I know it seems logical to talk about the GPU lightmapper in this Graphics Experimental Previews subforum, but it actually seems like an official thread about it is in a different forum, the Global Illumination subforum, and I suppose it does make sense to have it there.
I mention this to you also because that post has some info in about which GPU Unity chooses to use and some related matters.
edited to add - there is a thread in the 2018.3 beta forum as well (which includes a link to the other thread I mentioned above) : GPU lightmapper preview