To play with friends, subscribe to the collection, select Half-Life Co-op gamemode in the bottom right and just start hls00amrl map (Half-Life Resized Maps category) through Garry’s Mod menu. How do you get a half life source in GMOD? Open the “garrysmod” Folder, then open the “addons” folder.
However, you will need to purchase and download (possibly) Counter Strike: Source and Team Fortress 2 for a lot of the textures and other game objects. Technically – you do not need to purchase anymore games.