GalliumOS currently does not support UEFI, however it is possible to get it working with some time and effort. We opted to exclude UEFI support for GalliumOS because there is currently no acceptable UEFI payload for the target hardware of GalliumOS. There are currently some teams working on bringing coreboot UEFI payloads to Chrome hardware, and following the release of a working UEFI custom firmware, GalliumOS does plan to add UEFI support.
UEFI support is interesting to GalliumOS because it could bring better Bay Trail support (see Support/BayTrail), support for Braswell (see Support/Braswell), potential support for some ARM models (see Support/ARM), and potentially even support for other non-Chrome hardware that could be considered as possible targets for GalliumOS. UEFI support is also interesting because it could potentially reduce boot time and possibly even bring it very close to parity with ChromeOS.
GalliumOS currently has no specific plans for how we plan to implement UEFI, however it seems likely that we will go about it in a very different way from Debian or Ubuntu. UEFI allows booting the Linux kernel directly, without the need for any bootloader, which could save us a lot of boot time by skipping GRUB entirely.