If you can find Purina flock raiser I’d switch to it, I don’t know what all stock is so I can’t look up the nutritional percentages. Some all flock type feeds are great, others can be too low in one thing or another.
I don’t know where you are but If you have a tractor supply they usually carry Purina flock raiser as well as durvetb vitamin complex. They also have rooster booster poultry cell which is a multivitamin mix, also there’s this stuff which is great
https://www.tractorsupply.com/tsc/product/prairie-pride-vitamins-electrolytes-8-oz?cm_vc=-10005
Avoid the sav-a chick, it hardly has anything in it.
You might also try getting some small syringes “3 ml” in case you can’t get the little one to drink vitamin water, tractor supply also usually carries those, syringe feeding goslings is trickier than adults because they’re so little but sometimes it’s necessary.
It could be a congnital issue, if that’s the case there isn’t much that can be done, it could also just be vitamin deficiency. Sometimes if the mother was low on vitamins when she laid the egg the yolk can have only just enough nutrients to get through incubation but leaves the gosling vitamin deficient upon hatching.