After this you should see a contents page, click save and continue once again. Then under categories select server content, click save and continue. Select items that work together from the 'What kind of collection is this' option. Set a name for your collection and load a branding image if you have one, followed by a short description. On the right you'll see Create Collection, select this. You will then need to select Garry's mod, and then on the top click collections. You'll then want to go to community, and then workshop. The contents of your collection can then be downloaded in it's entirety when you start the server.

Workshop is a collection of addons from different developers that you're able to add to a collection. The Steam workshop is the easiest and quickest way to add lots of addons to your Garry's Mod server.