Launch the game; the mod will download automatically and appear in your menu.
To download any of these features, use the official Steam Workshop for Garry's Mod: Open the for Garry's Mod.
If you are looking for content related to "15," you may be looking for one of the following community-created items:
: A popular community map available on the Steam Workshop.
: A computer simulation addon for Garry's Mod that has various "fixed" versions available on the Steam Workshop.