I'll answer your questions in reverse order.
Gate is pretty close to an epic Summon Monster spell. There is also the epic summon seed which one may use to develop epic summoning spells.
Here is a list I saved from somewhere:
All Additional Summon Monster Targets Categorized
by Source
Monster Manual III
Demon, Arrow (Summon Monster VII, CE, summonable by Evil casters only, replaces babau)
Storm Elementals - Small (Summon Monster IV, TN), Med (Summon Monster VI, TN), Large (Summon Monster VII, TN), Huge (Summon Monster VIII, TN), Greater (Summon Monster IX, TN)
Monster Manual IV
Clockwork Mender (Summon Monster II, LN)
Demon, Nashrou (Summon Monster III, CE) - not listed in the table of contents as a summonable creature, but it is designated so in its text!
Dwarf Ancestor (Summon Monster V, LG, summonable by clerics of Moradin only)
Howler Wasp (Summon Monster II, CE)
Inferno Spider (Summon Monster VI, TN)
Justice Archon (Summon Monster VII, LG)
Windrazor (Summon Monster III, CE)
Windscythe (Summon Monster VI, CE)
Wrackspawn (Summon Monster IV, CE)
Yugoloth, Corruptor of Fate (Summon Monster VI, NE)
Yugoloth, Voor (Summon Monster IV, NE)
Monster Manual V
Arcadian Avenger (Summon Monster IV, LG, LG/LN caster)
Demon, Adaru (Summon Monster IX, CE, summonable by Evil casters only)
Demon, Carnage (Summon Monster IV, CE, summonable by Evil casters only)
Demon, Gadacro (Summon Monster IV, CE, summonable by Evil casters only)
Demon, Solamith (Summon Monster VII, CE, summonable by Evil casters only)
Devil, Gulthir (Summon Monster VI, LE, summonable by Evil casters only)
Fetid Fungus (Summon Monster II, summonable by clerics of Zuggtmoy only)
Book of Exalted Deeds
Bauriaur (Summon Monster III, CG)
Coure Eladrin (Summon Monster III, CG)
Musteval Guardinal (Summon Monster III, NG)
Cervidal Guardinal (Summon Monster IV, NG)
Rhek (Summon Monster IV, NG)
Leskylor (Summon Monster V, NG)
Equinal (Summon Monster VI, NG)
Asura (Summon Monster VII, CG)
Leskylor, three-headed (Summon Monster VII, NG)
Hollyphant (Summon Monster VIII, NG)
Lupinal Guardinal (Summon Monster VIII, NG) – From the Monster Manual II, but not noted as summonable there
Warden Archon (Summon Monster VIII, CG)
Firre Eladrin (Summon Monster IX, CG)
Moon Dog (Summon Monster IX, NG)
Owl Archon (Summon Monster IX, LG)
Quesar (Summon Monster IX, TN)
Sword Archon (Summon Monster IX, LG)
Ursinal Guardinal (Summon Monster IX, NG)
Complete Arcane
Elemental Grue (Summon Monster IV, NE) - Air, Fire, Earth, or Water.
Fiend Folio
Aoa Droplet (Summon Monster IV, TN)
Bacchae (Summon Monster III, CN)
Demodand, Farastu (Summon Monster IX, NE)
Demon, Skulvyn (Summon Monster IV, CE)
Demon, Wastrilith (Summon Monster IX, CE)
Deva, Monadic (Summon Monster IX, G)
Deva, Movanic (Summon Monster VII, G)
Formian, Armadon (Summon Monster VII, LN)
Formian, Observer (Summon Monster IX, LN)
Formian, Winged Warrior (Summon Monster VI, LN)
Imp, Bloodbag (Summon Monster IV, LE)
Imp, Euphoric (Summon Monster IV, LE)
Imp, Filth (Summon Monster IV, LE)
Kaorti (Summon Monster II, NE)
Maelephant (Summon Monster VIII, LE)
Nerra, Kalareem (Summon Monster IV, TN)
Nerra, Sillit (Summon Monster VI, TN)
Nerra, Varoot (Summon Monster II, TN)
Rilmani, Cuprilach (Summon Monster IX, TN)
Rilmani, Ferumach (Summon Monster VII, TN)
Slaad, Mud (Summon Monster VI, CN)
Yugoloth, Piscoloth (Summon Monster VII, NE)
Yugoloth, Skeroloth (Summon Monster IV, NE)
Fiendish Codex II
Abishai, White (Summon Monster IV, LE, LE caster, replace fiendish giant wasp)
Abishai, Black (Summon Monster V, LE, LE caster, replace bearded devil)
Abishai, Green (Summon Monster VI, LE, LE caster, replace chain devil)
Abishai, Blue (Summon Monster VII, LE, LE caster, replace bone devil)
Amnizu (Summon Monster VI, LE, LE caster, replace xill)
Kalabon (Summon Monster III, LE, LE caster, replace fiendish ape)
Orthon (Summon Monster VIII, LE, LE caster, replace hellcat)
Pain Devil (Summon Monster VI, LE, LE caster, replace chain devil)
Brachina (Summon Monster IX, LE, LE caster, replace fiendish dire shark)
Spined Devil (Summon Monster IV, LE, LE caster, replace fiendish giant wasp)
Steel Devil (Summon Monster VII, LE, LE caster, replace fiendish megaraptor)
Ghostwalk
Demon, Artaaglith (Summon Monster VI, CE)
Magic of Incarnum
Souleater (Summon Monster VII, NE, evil caster, replaces Huge fiendish monstrous scorpion)
Planar Handbook
Elementite Swarm (any) (Summon Monster VI, TN, replaces Elemental, Large) – 4 kinds.
Elsewhale (Summon Monster VIII, NG, replaces celestial cachalot whale)
Elysian Thrush (Summon Monster I, NG, replaces celestial owl or celestial monkey)
Energon (any) (Summon Monster V, TN, replaces Elemental, Medium) – and technically this allows the xeg-ya and xeg-yi from Manual of the Planes as well for a total of 7.
Gaspar (Summon Monster VIII, TN, replaces celestial triceratops or Colossal fiendish monstrous centipede)
Limbo Stalker (Summon Monster VIII, CN, replaces blue slaad)
Nightmare, Lesser (Summon Monster IV, NE, replaces fiendish dire wolf or howler)
Pack Fiend (Summon Monster VII, NE, replaces Huge fiendish monstrous scorpion)
Unraveler (Summon Monster IX, LN, replaces green slaad)
Ur' Epona (Summon Monster II, TN, replaces celestial riding dog)
Planar Handbook’s New Templates
A Celestial or Fiendish creature may instead become Anarchic (Chaotic) or Axiomatic (Lawful) at no spell level adjustment.
A Celestial creature may instead become a Vivacious creature (filled with positive energy for healing over time) for +1 spell level.
A Fiendish creature may instead become an Entropic creature (filled with negative energy for damage over time) for +1 spell level.
Races of Stone
Hammer Archon (LG, Races of Stone, summonable by a Dwarf, Gnome, or Goliath caster only)
Sandstorm
Glass mephit (Summon Monster IV, TN)
Sulfur mephit (Summon Monster IV, TN)
Stormwrack
Fiendish dire barracuda (Summon Monster V, NE, replaces fiendish dire boar)
Fiendish dire eel (Summon Monster IV, CE, replaces fiendish dire wolf)
Fiendish montrous diving spider (various, CE, replaces other fiendish monstrous spiders)
Fiendish sea snake (various, CE, replaces other fiendish vipers)
Caller from the Deeps (Summon Monster VIII, NE, replaces Elemental, greater)
Yugoloth, Echinoloth (Summon Monster VIII, NE, replaces blue slaad)
Fiendish monstrous crab, Small (Summon Monster I, NE, replaces fiendish monstrous scorpion, small)
Fiendish monstrous crab, Medium (Summon Monster II, NE, replaces fiendish monstrous scorpion, medium)
Fiendish monstrous crab, Large (Summon Monster V, NE, replaces fiendish monstrous scorpion, large)
Fiendish monstrous crab, Huge (Summon Monster VII, NE, replaces fiendish monstrous scorpion, huge)
Fiendish monstrous crab, Gargantuan (Summon Monster IX, NE, replaces fiendish monstrous scorpion, gargantuan)
Celestial archelon (Summon Monster VIII, NG, replaces celestial triceratops)
Fiendish icthyosaur (Summon Monster VI, NE, replaces fiendish rhinoceros)
Fiendish mosasour (Summon Monster IX, CE, replaces bebilith)
Celestial plesiosaur (Summon Monster IX, NG, replaces celestial roc)
There are no additional summonable creatures in BoD, Draconomicon, the Complete series up through Scoundrel/Mage/Champion, the Races series, DMGII, Exp. Psionic Handbook, Heroes of Battle/Horror, Libris Mortis, Lords of Madness, Manual of the Planes, or Unearthed Arcana.
