In order to edit the original game textures/models you'll need to first extract them using Crowbar.
Open the page below and scroll to the download section. Once downloaded, extract the archive into a folder.
Browse…
next to the Package field and find your file, see Common File LocationsBrowse…
next to the Output and select the output locationUnpack
Addon files can be found in two different locations depending on when the addon was created.
GarrysMod/garrysmod/addons
workshop/content/4000/<addon_id>
— you can get the <addon_id> from the workshop linkGarrysMod/garrysmod
garrysmod_dir.vpk
GarrysMod/sourceengine
hl2_textures_dir.vpk