Need help loading server maps?

Hello, I’m new to Garry’s Mod and have recently purchased it… I bought that with Counter Strike: Source, Left 4 dead 2, and Team Fortress 2…

For some reason whenever I want to join a server it says I cant CSC the map… I’m confused with the way this works? I can’t join any servers, anyone wanna help or point me to which maps to download… or give me a link or something? I own a Mac if that has to deal with any type of downloads ha I also run it off of steam…