So I just installed GT so I can join this one server. Sadly, I cannot use GMod while GT is running. I don't understand what is going on.

Hello,

That is unfortunate. GT Lite does have compatibility issues with some games on some PC's. Did GT Lite detect GMod? If so, you could uncheck all the monitoring/tracking features for it. These checkboxes are located in the "Game Setup" tab of the "Settings" window (Locate GMod and click edit).

Best regards.

I realize this is necroposting, but I've narrowed down to what the issue is with GameTracker crashing Garry's Mod.

I took the .mdump file and read it here.

Thread 20 of Garry's Mod launch is what crashes, crashing the entire game. It only happens when GameTracker is open. I don't know much about GSInGameClient.dll, so either GameTracker is interfering with thread 20, specifically GSInGameClient.dll, or GSInGameClient.dll is a GT DLL and needs to be fixed.