Installing custon content

Hey guys

me and a friend are currently hosting a gmod build server for the 1st time!

we are using filezilla to access our server but can anyone help us on how to add custom content please.

we added it to the server as you do for singleplayer but we soon learnt this wont work. someone said something about a download list but i got confused and decided you guys are the best to come to.?

so can anyone help us with adding custon content into our server?