I took a couple of semesters of University physics - where you derive the problem instead of plug and chug an equation - and never heard of a “gal”.
Must be the mech. eng. folks that use the term.
Physics jocks, actually. People who study gravimetry and variances in gravity on earth.
It is a “derived” unit, and not part of the SI system, and usually one doesn’t see them used outside of some CivE’s doing quake stuff and the aforementioned physics jocks.
Personally, I never understood the need for such a unit. I’m someone who likes to observe oddball units tho - eg, in the VAX/VMS operating system, the engineer who wrote the login code made the configurable login timeout in “micro-fortnights.” Most people have never heard of a “fortnight,” much less one-millionth of one.