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 locationUnpackAddon files can be found in two different locations depending on when the addon was created.
GarrysMod/garrysmod/addonsworkshop/content/4000/<addon_id> — you can get the <addon_id> from the workshop linkGarrysMod/garrysmodgarrysmod_dir.vpkGarrysMod/sourceenginehl2_textures_dir.vpk