Hello.

The tracking on my Garry's Mod server has been down for over 12 hours for no apparent reason. The DNS Name resolves fine for me.

Is there a list of IPs I need to unblock? Perhaps this was due to a recent DDoS on my server.

It does look possible that GT may be blocked. The following are the IPs you would want to check for:

208.167.241.190
208.167.241.185
208.167.241.186
108.61.78.147
108.61.78.148
108.61.78.149
108.61.78.150

When I did a test scan, GT appeared to have reached your server but was waiting for more data in the response from it. You may want to make sure your network is not overloaded, or perhaps a simple restart of the game server when you are able.