GUI is coming in 16. I may have to put my 6a on the beta to play with it. As of now I don't think there's is a way to use anything other than Debian, but I'm sure some tools will pop up once it's less experimental. Reading the docs it looks like you just need a kernel, initrd and a disk image to hand to the VM, but I didn't think there's a front end for that yet. It just came out in the March update, so it will take a bit maybe. 16 should be out by summer, will probably see more tools then.
Discussion
Well if we get GUI with 16 then you could install PHOSH and have a mobile interface. Lots of apps in Debian's repo have been updated to work on mobile and flatpak has even more. Being able to pass audio and data connection would be awesome.
GrapheneOS may have GUI and audio as soon as today!