Gel coat on rims breaking down

Not sure exactly what gel coat you are speaking of, but it sounds like you are talking about the the clear coat on the wheels. Depending on their condition, the best bet would be to clean, sand/prep, paint, and apply a fresh clear coat. If you wanted, you could probably just reapply the clear coat if the wheels are in good shape.

There are paints and clear coasts made specifically for wheels. Here is an example: https://duplicolor.com/product/wheel-coating