Hi @darstellungsart
from the UE 5.3 documentation NOT the 4.27? documentation
GPU Lightmass Global Illumination in Unreal Engine | Unreal Engine 5.3 Documentation
GPULM does not require the Virtual Texturing system to be enabled to simply build lighting but does require it for the interactive and real-time editing functionality. If you do not intend to use virtual texturing in your project or do not require interactive preview, you can forego enabling these and save some overhead while working in the editor.
Sorry about picky with version of documentation, but so much has changed between 5.1 and 5.3 documentation.
Lightmass compression is missed out completely!
Many developers, including me, have had problems with the early documentation. UE 5EA was the worst 