If you deal damage to a Gorilla, but it kills you, the Gorilla gains all its health back to max. I have tested it with both regular and Silverback Gorillas. This issue only effects the Silverback Gorillas.
Here are some images:
--Before death

--After death

Tested on latest mod and citadel version; This may be an issue more with Torohealth. I got a silverback down to 19 hearts with about 7 hits from a netherite sword, got killed by the gorilla, immediately spawned in my bed right next to it and killed it with only 2 hits from the netherite sword. Torohealth showed it as having 100 hearts at this point though.

Mod Version 1.6.0
Citadel Version 1.6.2
Minecraft Version 1.16.5
Forge Version 36.0.43