Check if your Garry's Mod server is online
Garry's Mod replies over the standard Source query protocol, so when the check fails it is nearly always a firewall and very rarely the game.
The ports Garry's Mod uses
Queries use UDP on the very port players join on, which makes this simpler than most games on the list: one port, one protocol. A TCP-only rule can look fine in a quick test and still fail every query.
Every port a default Garry's Mod install uses
With the game port left at 27015. If you changed it with -port, the other ports move with it.
| Port | What it is for |
|---|---|
| 27015 UDP | Game trafficRequired Gameplay and Steam queries share this UDP port. |
| 27015 TCP | Steam / HLTV handshakeRequired Source servers use the same number over TCP for parts of the Steam handshake. Forward both protocols. |
| 27020 UDP | SourceTVOptional Only if you run SourceTV spectating. |
Still offline? Try this
Open port, failed query: the addon collection is the next suspect. Until a big collection has finished mounting at boot, nothing replies — and a startup Lua error can leave the process alive but permanently unqueryable.
What a Garry's Mod server needs to run well
- Memory, 16 players
- 4 GB
- CPU cores
- 4+
- Disk
- 30 GB+
Addon Lua runs on the main thread, so a heavy addon list is a CPU problem as much as a memory one.