A singer I once heard (can't remember who - it was a long time ago) sang "Do tha let me go!" - presumably northern dialect for "Do thou ... etc". Of course "thou" is singular, and "gals" are plural - but then, relaxing the rules of grammar is an old folk tradition.

Or maybe the shanty-man was referring to a particular gal, the one mentioned in the following verse?

"She chased me round the parlour boys,wasn't that a show?
Hurrah me yaller gals, doodle, let me go!
She caught me by the bobstay lads, and wouldn't let me go!
Hurrah, me yaller gals, doodle let me go!"

By the bye, the first line of the chorus is sometimes sung as

"Doodle let me go, me gals - let me doodle go!"

Which seems to fit quite well with the sentiments of the verse quoted above.

Wassail!