There is an issue that I’ve submitted a ticket but got ignored, that is a deal breaker for GPULM.
If you have a stationary directional light (and static skylight), then the volumetric fog will be unshadowed everywhere, so you will see fog even in dark places.
With CPU Lightmass of course it works correctly
With GPU Lightmass and static directional light the fog works correctly, the problem is only with stationary light.
This happens both in UE4 4.27, and UE5 p2/github build.