Best place to advertise my new darkrp server?

Best thing to do would be to make your server stand out with the title - differ your server from the popular leading servers and remember that it takes time. You won't just attract players overnight. You need concurrent players to boost the player count, maybe some of your friends? Then they can invite their friends or something? Only then will people join your server off of the gmod multiplayer menu. Nobody wants to join an empty server. So what I'd suggest is to play with a bunch of your friends for a little while and surely enough, random people will start joining.

Hope this helps