how do you get people to join your gmod server

Have a good returning factor. Don't just give things away but make it easy enough to earn with a day or 2 of grinding. Have something to keep them coming back. Stay away from cookie cutter concepts like "hey man buy donor admin for x amount of money" or "This job is for VIP only". Don't create a monarch..make everyone equal.
Running a DarkRP server is SUPER hard. It's takes alot of money to hire coders to do stuff for you unless you want a generic, scriptfodder created server. The reason Zarp and ICE are the top servers is because they don't use anything from workshop or scriptfodder. Everything is coding FOR them.