Alloing, in the south

Do I understand correctly that gadael, to leave, also means to allow, or let?

I heard something like the word gadael (mutated to adael after a vowel) in the third lesson of the third course.

It does, David…in the sense of to “let/leave”

Diolch ynfower.

A nice little idiom round here is “Gadwch y fod” = “Let it be / Leave it alone”

In “Symud 'Mlaen” by the wonderful Elin Fflur, a line in the chorus is “Gad fi fod”, let me be. Then there’s “Gad i mi feddwl…” (Allow) Let me think.
Following from that, you could say “Gad i mi 'neud e” - let me do it.
Notice the soft mutation of the following verb.