What makes a GMOD server good?

Personally, a few factors go into making a good Garry's Mod server.

Don't be one of those owners who purchase all the content from GMODStore, and call it a unique server. Get something unique coded for the server, hire a developer. Add more originality to your own server, if you're just whipping addons in that you purchase, the server won't be unique and for sure will end up lagging to due to so many different coding styles and how storage happens.

Good staff and fair rules. Many servers let abusive people into their staff team or have rules that are fairer to certain ranks than just normal players.

Most important. Make sure to communicate with the player base and listen to feedback! This is the key thing most servers lack, is communication with their own player base. Listening to feedback is amazing, it shows your players you care, and it doesn't mean everything they say needs to be done but listen to them. Make sure you tell them what is in the works, or tell them you're working on something. Don't be working on a project for 2 months with no information to the public, they'd think you'd either stop caring for your server or are just not active.