Yes, these are in UTPlayerController.
Only does a “WorldInfo.ForceGarbageCollection(bool);”, I have tested this function manually, and only does a CG in that time.
But it does not prevent it from doing it every 30 seconds. I think that these lines are to force a GC in some moments, for example when you enter or exit from a menu or pause, to minimize the hitching.
According to the Epic documentation, it is impossible to avoid GC, but you have to try to have the minimum number of objects to check so that the peak is not too high.
Currently I have about 300,000 objects in the map, the peak is about 45 ms. But in a blank map with UTgame there are 115,000 objects and the peak is 16 ms, for only a floor and a cube meshes (was a surprise for me to see the high number of objects in the world).