Cleaning Gmod

Can someone tell me how to ‘clean’ or get rid of things you have downloaded because i want to start again.

Go to your steam folder and inside your username folder (should be like steam/steamapps/username) there’s a garrysmod folder, just delete it and run the game, the folder will be recreated. If you want to save addons then just copy the ones you want.

Here’s another question: If I delete local content from, say, CS:S on steam, will I still have the source files there for Garry’s mod?

No,if you remove CS:S from steam,it will remove the GCFs and the content won’t be there again.