Locate the Addon:

Find the addon containing the 2D NPC in your garrysmod/addons folder.

Extract the Files:

Use a tool like GCFScape to open the .gma file of the addon.

Extract the contents to a new folder.

Identify the NPC Files:

Look for the model files (.mdl, .vtx, .phy, .vvd) in the models folder.

Find the texture files (.vtf, .vmt) in the materials folder.

Locate the Lua script defining the NPC in the lua folder.

Decompile the Model:

Use Crowbar to decompile the .mdl file into a format that can be edited in Blender or another 3D modeling software.

Edit the Model:

Open the decompiled model in Blender.

Make any necessary changes to the model or textures.

Recompile the Model:

Use Crowbar to recompile the model back into a format that GMod can use.

Test the NPC:

Place the modified files back into the appropriate folders in your garrysmod/addons directory.

Launch GMod and test the NPC to ensure it works correctly.