This is referred to as a "rough navel" -- it looks worse than it is in most cases. IIRC, it is due to too high of humidity during hatch.
When I've had this happen, I dust the navel with both styptic powder (dries it up) and tetracycline powder (helps prevent infection). They always heal up in a day or two. Repeat 2 or 3 times a day for 2 or 3 days at least.
The most important thing is to make sure that they goslings aren't running around on any sharp surface or object such as wire mesh -- if something cuts or pierces the navel, they can bleed out a lot. If you incubator has a wire mesh bottom, I'd remove the goslings and put them in a brooder with a soft, non-slip surface that is easy to clean, such as old towels that you can change out 3 or 4 times a day.
They should be fully healed up in about a week.