So Hell's got the devils and the Abyss has demons. Poor Gehenna got all left out (well, Manual of the Planes says that the yugoloths moved in, but that doesn't answer the question of what really lives there).
So, I made some inhabitants up. Gehenna petitioners are immune to poison and acid, and have fire and cold resistance 20, so I went from there.
I called them "Ghulsha" and made three varieties. All just have a physical description, no society/ecology stuff (since the adventure consisted of the PCs going to Gehenna to whup some ass, and there wasn't much of that kind of detail needed). Also, I made them up straight in stat-block format instead of the MM style. They're free, so no complaining.
All have standard treasure, and act like your usual lawful evil outsiders. Tricky, organized, and evil.
Consider these copyrighted, but OGL.
So, I present for the infliction of anguish on your own PCs, the Ghulsha:
#1: Kluurgoth. Tall, slender, black-scaled winged humanoid, vaguely reptilian; abnormally long arms; needle-like claws and teeth; surrounded by wispy red-orange vapors. Fly in packs, use their acid fogs to blanket whole areas.
Kluurgoth: CR 11; Large Outsider (lawful, evil); HD 12d8+60; hp 114; Init +0; Spd 40 ft, fly 50 ft (average); AC 23 (-1 size, +14 natural); Atk +20 melee (2d8+8 plus vampiric wound, bite), +15 melee (1d6+4 [x2], 2 claws); Reach 10 ft; SA spell-like abilities, vapors; SQ damage reduction 20/+2, SR 22, ghulsha qualities, see in darkness; AL LE; SV Fort +13, Refl +8, Will +11; Str 27, Dex 11, Con 20, Int 13, Wis 17, Cha 16.
Skills: Climb +20, Hide +8, Intimidate +15, Intuit Direction +15, Jump +20, Listen +15, Move Silently +12, Search +13, Spot +15.
Feats: Cleave, Power Attack, Improved Bull Rush, Sunder.
Ghulsha: Immune to poison and acid. Fire and cold resistance 20.
SA--Spell-like Abilities (Sp): At will: desecrate, detect good, detect magic, detect thoughts, produce flame. 1/day: acid fog. As 12th level sorcerer (save vs. DC 13+spell level).
SA--Vapors (Ex): Continually surrounded by acidic, choking vapors. All non-ghulsha in a 5 ft radius take 1d4 acid damage per round and make a Fortitude save (DC 21) each round or be only able to take a partial action on their following turn from choking.
SA—Vampiric Bite (Su): Bite hits heal the kluurgoth for half the damage caused to living creatures.
SA—Wound (Su): Bite hits cause bleeding wounds that cause 2 hp damage per round until bound or magically healed (Heal DC 10).
SQ—See in Darkness (Su): Can see in darkness of any kind.
#2: Vaadroth. Tall, muscular, hunched ape-like humanoid wielding a black greatsword; fur striped red and dark green; huge bat wings; long tail with stinger. Small groups (party-sized), like to beat on PCs with their swords.
Vaadroth: CR 16; Large Outsider (lawful, evil); HD 17d8+85; hp 161; Init +6; Spd 40 ft, fly 60 ft (average); AC 28 (-1 size, +2 Dex, +17 natural); Atk +25/+20/+15/+5 melee (2d8+13, greatsword), +20 melee (2d4+5 plus poison, stinger); Reach 10 ft; SA spell-like abilities, breath weapon; SQ damage reduction 30/+3, SR 27, ghulsha qualities, see in darkness; AL LE; SV Fort +15, Refl +12, Will +14; Str 28, Dex 14, Con 21, Int 16, Wis 18, Cha 19.
Skills: Bluff +19, Climb +23, Concentration +20, Diplomacy +19, Hide +12, Intimidate +19, Intuit Direction +18, Knowledge (planar) +17, Listen +18, Move Silently +16, Search +17, Spellcraft +18, Spot +19.
Feats: Cleave, Great Cleave, Power Attack, Improved Initiative, Sunder.
Ghulsha: Immune to poison and acid. Fire and cold resistance 20.
SA--Spell-like Abilities (Sp): At will: blur, desecrate, detect good, detect magic, detect thoughts, minor globe of invulnerability, produce flame, summon swarm. 1/day: greater dispelling, circle of death. As 17th level sorcerer (save vs. DC 14+spell level).
SA--Poison (Ex): Poisonous stinger, Fortitude save (DC 23), initial & secondary damage 1d4 temp Con.
SA—Breath Weapon (Su): 3/day can breathe a 10-ft cone of acid for 4d8 points of acid damage, Reflex save (DC 23) for half.
SQ—See in Darkness (Su): Can see in darkness of any kind.
Possessions: huge greatsword.
#3: Zaasoth. Tall, gargoyle-like winged humanoid with blackish-silver scales that shimmer with opalescent colors. The smart ones. They try to order around the vaadroth and kluurgoth.
Zaasoth: CR 16; Large Outsider (lawful, evil); HD 17d8+17; hp 93; Init +7; Spd 40 ft, fly 60 ft (good); AC 26/27 (-1 size, +3 Dex, +14 natural, Dodge); Atk +21 melee (2d4+5 [x2], 2 claws); Reach 10 ft; SA spell-like abilities; SQ damage reduction 30/+3, SR 27, ghulsha qualities, see in darkness; AL LE; SV Fort +11, Refl +13, Will +17; Str 20, Dex 17, Con 13, Int 22, Wis 20, Cha 23.
Skills: Bluff +22, Climb +20, Concentration +20, Diplomacy +22, Gather Information +22, Hide +14, Intimidate +22, Intuit Direction +20, Knowledge (planar) +22, Listen +21, Move Silently +18, Search +22, Sense Motive +20, Spellcraft +22, Spot +21.
Feats: Iron Will, Dodge, Mobility, Improved Initiative, Spell Penetration.
Ghulsha: Immune to poison and acid. Fire and cold resistance 20.
SA--Spell-like Abilities (Sp): At will: color spray, entropic shield, greater dispelling, hold monster, magic missile, rainbow pattern, shatter, unholy blight. 1/day: flesh to stone, spell turning, repulsion, prismatic spray. As 17th level sorcerer (save vs. DC 16+spell level).
SQ—See in Darkness (Su): Can see in darkness of any kind.
I used these for an 18th level group. Worked fairly well. I even used a 12th level cleric kluurgoth and a 7th level sorcerer zaadroth, for more fun.
So, if you're off to Gehenna, now you know what to look out for.
So, I made some inhabitants up. Gehenna petitioners are immune to poison and acid, and have fire and cold resistance 20, so I went from there.
I called them "Ghulsha" and made three varieties. All just have a physical description, no society/ecology stuff (since the adventure consisted of the PCs going to Gehenna to whup some ass, and there wasn't much of that kind of detail needed). Also, I made them up straight in stat-block format instead of the MM style. They're free, so no complaining.

All have standard treasure, and act like your usual lawful evil outsiders. Tricky, organized, and evil.
Consider these copyrighted, but OGL.
So, I present for the infliction of anguish on your own PCs, the Ghulsha:
#1: Kluurgoth. Tall, slender, black-scaled winged humanoid, vaguely reptilian; abnormally long arms; needle-like claws and teeth; surrounded by wispy red-orange vapors. Fly in packs, use their acid fogs to blanket whole areas.
Kluurgoth: CR 11; Large Outsider (lawful, evil); HD 12d8+60; hp 114; Init +0; Spd 40 ft, fly 50 ft (average); AC 23 (-1 size, +14 natural); Atk +20 melee (2d8+8 plus vampiric wound, bite), +15 melee (1d6+4 [x2], 2 claws); Reach 10 ft; SA spell-like abilities, vapors; SQ damage reduction 20/+2, SR 22, ghulsha qualities, see in darkness; AL LE; SV Fort +13, Refl +8, Will +11; Str 27, Dex 11, Con 20, Int 13, Wis 17, Cha 16.
Skills: Climb +20, Hide +8, Intimidate +15, Intuit Direction +15, Jump +20, Listen +15, Move Silently +12, Search +13, Spot +15.
Feats: Cleave, Power Attack, Improved Bull Rush, Sunder.
Ghulsha: Immune to poison and acid. Fire and cold resistance 20.
SA--Spell-like Abilities (Sp): At will: desecrate, detect good, detect magic, detect thoughts, produce flame. 1/day: acid fog. As 12th level sorcerer (save vs. DC 13+spell level).
SA--Vapors (Ex): Continually surrounded by acidic, choking vapors. All non-ghulsha in a 5 ft radius take 1d4 acid damage per round and make a Fortitude save (DC 21) each round or be only able to take a partial action on their following turn from choking.
SA—Vampiric Bite (Su): Bite hits heal the kluurgoth for half the damage caused to living creatures.
SA—Wound (Su): Bite hits cause bleeding wounds that cause 2 hp damage per round until bound or magically healed (Heal DC 10).
SQ—See in Darkness (Su): Can see in darkness of any kind.
#2: Vaadroth. Tall, muscular, hunched ape-like humanoid wielding a black greatsword; fur striped red and dark green; huge bat wings; long tail with stinger. Small groups (party-sized), like to beat on PCs with their swords.
Vaadroth: CR 16; Large Outsider (lawful, evil); HD 17d8+85; hp 161; Init +6; Spd 40 ft, fly 60 ft (average); AC 28 (-1 size, +2 Dex, +17 natural); Atk +25/+20/+15/+5 melee (2d8+13, greatsword), +20 melee (2d4+5 plus poison, stinger); Reach 10 ft; SA spell-like abilities, breath weapon; SQ damage reduction 30/+3, SR 27, ghulsha qualities, see in darkness; AL LE; SV Fort +15, Refl +12, Will +14; Str 28, Dex 14, Con 21, Int 16, Wis 18, Cha 19.
Skills: Bluff +19, Climb +23, Concentration +20, Diplomacy +19, Hide +12, Intimidate +19, Intuit Direction +18, Knowledge (planar) +17, Listen +18, Move Silently +16, Search +17, Spellcraft +18, Spot +19.
Feats: Cleave, Great Cleave, Power Attack, Improved Initiative, Sunder.
Ghulsha: Immune to poison and acid. Fire and cold resistance 20.
SA--Spell-like Abilities (Sp): At will: blur, desecrate, detect good, detect magic, detect thoughts, minor globe of invulnerability, produce flame, summon swarm. 1/day: greater dispelling, circle of death. As 17th level sorcerer (save vs. DC 14+spell level).
SA--Poison (Ex): Poisonous stinger, Fortitude save (DC 23), initial & secondary damage 1d4 temp Con.
SA—Breath Weapon (Su): 3/day can breathe a 10-ft cone of acid for 4d8 points of acid damage, Reflex save (DC 23) for half.
SQ—See in Darkness (Su): Can see in darkness of any kind.
Possessions: huge greatsword.
#3: Zaasoth. Tall, gargoyle-like winged humanoid with blackish-silver scales that shimmer with opalescent colors. The smart ones. They try to order around the vaadroth and kluurgoth.
Zaasoth: CR 16; Large Outsider (lawful, evil); HD 17d8+17; hp 93; Init +7; Spd 40 ft, fly 60 ft (good); AC 26/27 (-1 size, +3 Dex, +14 natural, Dodge); Atk +21 melee (2d4+5 [x2], 2 claws); Reach 10 ft; SA spell-like abilities; SQ damage reduction 30/+3, SR 27, ghulsha qualities, see in darkness; AL LE; SV Fort +11, Refl +13, Will +17; Str 20, Dex 17, Con 13, Int 22, Wis 20, Cha 23.
Skills: Bluff +22, Climb +20, Concentration +20, Diplomacy +22, Gather Information +22, Hide +14, Intimidate +22, Intuit Direction +20, Knowledge (planar) +22, Listen +21, Move Silently +18, Search +22, Sense Motive +20, Spellcraft +22, Spot +21.
Feats: Iron Will, Dodge, Mobility, Improved Initiative, Spell Penetration.
Ghulsha: Immune to poison and acid. Fire and cold resistance 20.
SA--Spell-like Abilities (Sp): At will: color spray, entropic shield, greater dispelling, hold monster, magic missile, rainbow pattern, shatter, unholy blight. 1/day: flesh to stone, spell turning, repulsion, prismatic spray. As 17th level sorcerer (save vs. DC 16+spell level).
SQ—See in Darkness (Su): Can see in darkness of any kind.
I used these for an 18th level group. Worked fairly well. I even used a 12th level cleric kluurgoth and a 7th level sorcerer zaadroth, for more fun.
So, if you're off to Gehenna, now you know what to look out for.
