Check your ~/Library/Logs/Diagnostic Reports folder (the root one, not the user one). And look for GPU reset/restart logs. I was experiencing a similar issue (Radeon Vega 48, 8GB). Resetting NVRAM helped. I suspect it is related to the driver problems in Catalina. Hopefully, it gets fixed in 10.15.2 (supposedly a similar issue that occurred when a second monitor is connected was fixed in Beta 2).
Setting shaders on Medium (other Graphics settings don’t matter too much) also helps with this and the Sky Temple crash/kernel panic.
My original post: Game Freeze and 'GPU Reset' in MacOS 10.15.1 - #4 by CatwalkGhost-2672