1. Is it still starting?
A new server downloads about 7 GB, and then its Workshop collection, before it registers with Steam. On Flux, the dashboard says when it is up; give a first start a few minutes.
2. Connect by address
Open the console and type connect followed by the address and port from the dashboard. If that works, the server is up and the rest of this list is about the browser.
3. The gamemode filter
Garry's Mod's browser groups servers by gamemode. A DarkRP server is under DarkRP, not under Sandbox.
4. The exact name
Search for the name as the server has it. If server.cfg sets hostname, that is the name, not the one on the launch line.
5. Steam's ranking
A server without a game server token logs into Steam anonymously and Steam ranks it lower. Create a token for App ID 4000 at steamcommunity.com/dev/managegameservers and set it (+sv_setsteamaccount, or the Steam Game Server Token field on Flux).
6. Password and region filters
Players hiding passworded servers do not see one with a join password, and a region filter hides servers filed under another sv_region.
Frequently asked questions
Why is my Garry's Mod server not in the server list?
Usually a filter: the gamemode, a region or hidden passworded servers. Connecting with the address in the console tells you whether the server itself is up.
Does a Garry's Mod server need a Steam token?
No; without one it logs in anonymously and works, but Steam ranks it lower in the browser. A token for App ID 4000 is free.