They were in use in England at that time, as documented in An Essay on Shooting, by Wm. Cleator.
"We have subjected this matter to the test of experiment, and the result has accordingly been, that a barrel of 22 or 24, which is the largest caliber usually employed in fowling pieces, threw its shot as closely as one of the smallest caliber, viz of 30 or 32."
In this instance he is using the term caliber in the same way we use gauge, and defines it as balls to the pound.
I have one reference to their use in this country, an early one, 1733.
"THE SOUTH CAROLINA GAZETTE
January 27, 1733
Charleston, South Carolina
JUST imported, and to be sold very cheap, by Wm. Lasserre, at his Store at Mr. Lloyd's on the bay.... broad cloths, bullets of 24 and 28 in the pound , gunpowder, &c."
Of course "28 in the pound" is the same as 28 gauge. At that time chances are very high that the bullets were for a smooth-bore gun, probably a fowler.
Spence