Advertising can be split into two types , indirect and direct.
Indirect advertising is where someone has for example a CSGO Gambling site in their name , this really doesn't deserve a straight permanent ban as they are not directly telling people to check out the site and it doesn't really have an effect on the server as it is not a Garry's Mod community. A simple warn and kick can do the job for this , a ban if necessary.
What you are talking about is when someone comes on the server and has no intention to play on the server and only wants to advertise their own server for their own benefit. So for example someone joins and says "Join ******* it's the best garry's mod server" blah blah blah. That's directly telling people to join their server and this I agree would warrant a permanent ban.