It's ubuntu based, but they use their own kernel. So you rely on their kernel builds, which is not ideal. If your device is one that works well with upstream kernel, you should just use a regular distro (or switch your galliumos kernel to track the regular ubuntu kernel).
Galliumos is no longer under development see r/galliumos for more info. Ubuntu 18.04 works great, see my other link in this for how to enable media keys.
It seems like they are still active, although not quite as much. The stickied post on that sub says so at least. The main draw for galliumos is support for odd devices for which upstream support is lacking. That's kind of why it exists, and I don't think things have changed materially for a lot of these old devices. Going forward, things should be a bit better since chromeOS itself tracks LTS kernel releases now, and I think they also plan to do kernel upgrades over the life of a device.
I'm on an older Toshiba Chromebook 2 and have been running Gallium for a while. I did some distro hopping recently and found that Manjaro, Ubuntu, and some others work almost perfectly out of the box. Bring in the Gallium keyboard layout and its great.
The work the Gallium team did was necessary, and its definitely paid off. It's nice to see things open up over the past few months.
