@ManuEx2 as long as you see that window with the title “GPU Lightmass Progress”, you are using GPU lightmass to bake. If you don’t see that window a little bit after you start baking, then probably you are using the regular lightmass.
Your CPU is still going to be used for some calculations, so don’t worry about it, its not like its switching between CPU and GPU baking, it is just that GPU lgihtmass also needs the CPU to do some things.