Have been using GPU LightMass for a while but still unclear about one thing.
Are the settings in World Settings-LightMass Settings ignored when GPU Lightmass is used?
Yes, the world settings lightmass settings are reserved to CPU baking only.
Source : How to Make Unreal Engine 4 Render Lighting Faster! - UE4 GPU Lightmass Tutorial - YouTube
Lightmap compression and the volumetric lightmap settings work
Otherwise everything else is pretty much ignored, both in the world settings and object settings, and some light settings.