Replying to keep this thread alive, as people are confusing Epic’s GPU Lightmass, with Luoshuang’s.
My understanding is that Epic’s version is really a preview render. Fast but noisy, unless you give it ridiculous samples.
However, given enough samples and time, it does produce a nicely detailed map. And of course, it’s a great way to evaluate light map resolution before sending it off to cpu lightmass.