Q,
From whence cometh your c1770?
My copy of Rox Vol 7 p775 has the 4 Choice Carols of which GBYMG is the first. It is reprinted from the Rox Coll itself at III, 452 with a duplicate at BL 11621, 1. 2, fol. 13., neither with colophon so difficult to date precisely.

The description on p777 at the foot of the reprinted broadside runs thus..
'No Colophon. Three woodcuts: etc...., Roxb. has no colophon, but the second and third of these ditties were printed by J Millet in a volume of New Carols (Wood's No 40), in Black-letter, c1674, or earlier, and this is the probable date of the originals. Ours is a modern reprint in White-letter. It is a fairly trustworthy version.'

This seems to me to be implying that though their copy is a modern reprint, it is a reprint of a broadside of the late 17thc, or not!