Lightmass is a CPU renderer. Building lighting requires loading the entire scene into memory though and most GPU’s dont have much memory, even the most expensive ones might go up to 12GB which isn’t that much for building a game level. For the vast majority of people they would not be able to take advantage of a GPU renderer. Also, it wouldn’t necessarily be faster, since waiting for the noise to clear up enough would still take quite a while. GPU renderers are good if you have multiple GPU’s and your scene fits in memory but otherwise the limitations are going to be an issue.