It's one of the LVM2 commands. They mostly start with lv for commands which deal with logical volumes, pv for commands which deal with physical volumes, and vg for commands which deal with volume groups.
Strictly, I guess vgs would be the most correct command to use to see how much available space there is, but Gaia only has the one volume group and only the one physical volume by default, so they return equivalent data.
NOTE: there are a bunch of commands in this family which change things. They will let you mess up your system spectacularly. Be really careful with any of these commands. Try them in a lab before you try them on a firewall or management which you care about. A bunch of the commands just show information like lvs, lvdisplay, pvs, pvdisplay, vgs, and vgdisplay. Those should be fine to run.
I still wish we could use ZFS instead of XFS and LVM2. Oh well.