Doesn't sound very promising, but you may be able to wrap the gosling carefully with gladwrap (cling film) to keep the yolk sac clean and moist, and keep the bird warm. If it is going to absorb the yolksac, it will assist. If it continues to deteriorate, consider humane dispatching. It sounds as if it should have stayed in its shell for a few days more. Birds will sometimes open a breathing hole, even if not ready to completely pip. It's important not to be tempted to help them out at this stage, as yolk absorption and absorption of blood within the membrane, will continue to be used up. There are some circumstances where I will help out a chick that is genuinely stuck, but that does take skill to recognise when to help.
If he does survive, he may always be weak.