Yep, angel wing. Get some vetwrap, or even bandaging and some tape, and tuck the feathers up under the wing where they should be, then bandage or wrap them in place by wrapping around the sturdier part of the wing. It may be a bit late to correct, but you can always try it. Don´t have it too tight, but do it tight enough not to slip off and hold the feathers in place, leave it like that for at least 5 days. Take it off, see how it goes. If it´s still flopping, try it again. I hope it works. Beautiful goslings, btw.