Even if only gpu is selected, at one point your cpu may start rendering. That's due your gpu memory being full.
At that point you either continue with cpu or close Daz and restart
Now you need to tick the OptiX Prime Acceleration box on in advanced settings
Also, Make sure you have the latest Nvidia driver fully installed with everything. If you use the default driver windows gives it does not install CUDA
OptiX is buggy. On my rig it forces the render onto CPU instead of using the GPU and takes Much longer. Might not be that way with every card but on mine it is the case
Lots of things aren't supposed to happen with Daz, but do. LOLIn Daz forum they say it was not supposed to happen, but it did happen, so.
I am having a similar problem, when I render without Iray, everything works, other than the iray only colors/textures, if I render with the cpu, everything works (though that defeats the point of buy a new video card), but if I use my brand new video card, then I get an empty render. Everything was fine (albeit slow), with my 970, but with the 2080, I can't get it to work.