Your right there, I had to look up one "what is a gill". I just asked my grandkids, and they had no idea where we got yards, inches etc. from.
The yard originally came from the width of cloth a weaver could weave on a hand loom. Basically it was the distance from one elbow to the other with the weaver's fingers touching in front of him.
And the inch comes from the width of a man's thumb.
The foot? - That needs no explanation.