Hi, and welcome to BYC! I'm very sorry to hear your gosling isn't doing well.
Chick starter isn't really made with waterfowl in mind, so it is definitely possible that it could be a vitamin deficiency. I would pick up the Nutri Drench and add it to their water. You may also be able to find something called Vitamin B Complex. I know
Tractor Supply carries it, I don't know if Atwoods does. That can be given right to the little guy, a few drops a day right into his beak, being careful to let him swallow it on his own so he doesn't aspirate. You could do the same with the Nutri Drench, but the B Complex is more powerful.
Additionally, if you can find a Flock Raise or All Flock type food, I'd switch them to that. Or if you can find Naturewise Meat Bird, that is what I feed my ducklings and goslings and they do very well on it.
Can you also give us more info on their brooder? What temperature you are keeping it, what you are using for bedding, etc, just in case?
Also, just as a little note, the feed store told you the wrong names for the breeds

The yellow one is probably a white Chinese. The other one, well, there's no such thing as an "African Chinese". He is probably either a brown Chinese, which is the same as the white Chinese, just a different color, or an African, which is an entirely different breed.