Since I just switched to using GalliumOS, I want to know if asking questions about distros that are not Ubuntu, but are still based on Ubuntu are relevant. GalliumOS is based on Ubuntu 15.10, so would it still be okay to ask questions about GalliumOS (or any other Ubuntu-based distribution) on the Ask Ubuntu website?
No, it is not okay, as such questions are offtopic.
Non-official Ubuntu Derivatives, including Mint, ElementaryOS, and GalliumOS, are offtopic on Ask Ubuntu. This has always been the case, and remains the case.
This is also explicitly stated in the Help center:
Questions that you should avoid:
Linux Mint, Elementary OS, Backtrack, Gnome-Remix (prior to 13.04), budgie-remix (16.04 and 16.10) and other Linux distributions (try our friends at Unix & Linux Stack Exchange).
Note that by "official" Ubuntu derivatives, we mean officially-recognized derivatives by the Ubuntu project - Ubuntu, Kubuntu, Lubuntu, Ubuntu GNOME, etc.