The installer insists on erasing the entire disk, not sharing with other partitions.
I removed the SSD for safety, and plugged in a second USB stick.
The installer took from 21:20 to 7:30 am the next day. It needed to be connected to wired Thunderbolt Ethernet, which in my case means in the laundry.
Downloading packages took a long time (about 2 hours). "grafting" took longer. Finally it failed to set up the bootloader. No big deal, try GRUB or rEFIt, right? But they didn't detect it.
There is an "EFI Boot" option in the startup menu (holding option). That just boots to a black screen.
I'm sorry, but issues like this mean that I think GUIX is not desktop-ready for bare metal. It'll be relegated to a virtual machine (along with OpenIndiana, PureOS, Haiku, Hello, etc).