Cleon
Legend
Well, compared to our chimp, 4HD and Con 14-16 seems like a reasonable place to start.
OK, I'd start by Advancing our chimp to Large and giving it 4 Hit Dice.
I'd go for Con 14 since (a) I don't believe real-world gorillas are any more disease resistant than chimps and (b) that's what the SRD Ape does. I'd also make its Slams the primary attack, for similar reasons.
Oh, and if I remember correctly adult gorillas aren't very fast climbers, so I'd drop its Climb speed to 10.
Since it's got AC6 in the 2E version, I'll top up its Natural Armour to match.
We end up with something very similar to the SRD Ape.
Are we leaving it its Bonus Multiattack, or do we make the big ape pay for it?
Oh, and I thought we'd reluctantly decided to drop the Chimp's Int to 2, but it's still Int 6 in the stats.
Suggested Gorilla:
Gorilla
Large Animal
Hit Dice: 4d8+8 (26 hp)
Initiative: +1
Speed: 30 ft. (6 squares), climb 10 ft.
Armor Class: 14 (-1 size, +1 Dex, +4 natural), touch 10, flat-footed 13
Base Attack/Grapple: +3/+14
Attack: Slam +7 melee (1d6+6)
Full Attack: 2 Slams +7 melee (1d6+6) and bite +5 melee (1d6+3)
Space/Reach: 5 ft./5 ft.
Special Attacks: Fierce grappler
Special Qualities: Low-light vision, poor swimmer, scent
Saves: Fort +6, Ref +5, Will +2
Abilities: Str 23, Dex 13, Con 14, Int 6, Wis 12, Cha 7
Skills: Climb +13, Listen +5, Spot +5, Survival +4
Feats: Alertness, Multiattack (B or regular?)
Environment: Warm forests
Note: I put its extra 2 skill points in Listen & Spot.