OK, NWN modding crash course:
Creature appearances have models and textures, and are listed in a 2da file. The game has a default appearance.2da, which may be overridden by a custom appearance.2da installed by the player or by an appearance.2da that is attached to a module hak.
The 2da contains rows listing the individual creature appearances. In the toolset, a creature or creature blueprint links to a row in appearance.2da by number to find the model and various other settings. So, if the 2da entry is missing, the toolset won’t be able to find the creature appearance, and if no creature in the module is using the row number of the appearance, you’ll never see the appearance ingame either.
To make things that use 2das be available at the same time in any given module, you need to merge the 2das. Take a look at this tutorial: Ryuujin’s Hakpack and 2da Merging Tutorial.
In and about NWN, the word “mod” is generally taken to stand for “module”. To avoid confusion going forward, I’d recommend referring to game modifications using the term for the specific type of modification it is - such as hak, override, model, reskin, sound, music, creature appearance, bodypart, armourpart, script, etc.
So, yep. What’re you trying to do with the gnoll appearances? If you want to replace the existing ones, you’ll want to open appearances.2da and change the model name on the rows for the existing gnoll creature appearances. If you want to make them available for use in a custom module, you’ll just want to add their rows to your custom appearance.2da.
Linking to NWN Explorer Reborn for extracting game files. Not sure this works for the EE. If it doesn’t, @Proleric’s uploaded the 1.69 level 2das here.