This page describes GalliumOS as a VirtualBox guest. For running VirtualBox on a system running GalliumOS, see VirtualBox Host.
GalliumOS does not support VirtualBox at all. Booting the GalliumOS installation media inside VirtualBox will lead to a very much non-functional system. Trust us, we know, we've tried.
That said, User:ColtonDRG has put together a VirtualBox appliance that you can install. This appliance is for software development purposes only, and should not be considered as an accurate representation of the GalliumOS experience. Also note that this is completely unofficial, and unrelated to the GalliumOS Project. ColtonDRG's GalliumOS VirtualBox Appliance