Folders

Open your steam library, right-click Garry's Mod, go to Properties > Local Files > Browse Local Files

Next go to garrysmod > data.

Here you will find all folders related to PAC, look below for more information.

All paths below are relative to the data folder so /pac3 is <gmod_folder>/garrysmod/data/pac3

/pac3	The main PAC folder, all you outfits are kept in here.
/pac3_cache	The cache folders, it contains cached models and .zip downloads. Deleting this folder will clear the cache.
/pac3/__animations	Custom animations folder, contains all the saved animations.
/pac3/__backup	Backup folder, contains backups automatically created by the editor.
/pac3/__backup_save	Save backup folder, contains backups created while overwriting saved outfits.

Keep in mind this only covers the latest version, your folder structure might be a bit different if you're using an older version.

Finding the data folder