Save a chick only has B12, while that is important the issue is that goslings can be deficient in more than just B12, other B vitamin deficiency or a vitamin E deficiency could cause the symptoms you’re seeing.
I would get a B vitamin mix like Miss Lydia suggested or poultry nutrient-drench.
There are other possible causes for nuerological symptoms like this besides vitamin deficiency, botulism or metal poisoning to name a few.
How long has he been tipping onto his back?
What do his droppings look like?
Have you noticed any other symptoms?
Could he have gotten into any compost, dirty stagnant water, or swallowed/been chewing on metal or paint?