Not sure of any non-formula milk that can aid in weight gain…but for young children, definitely don’t give them any low fat milk.
FYI:
Babies and toddlers need fat in their diets for a variety of reasons, including healthy brain development. So it's usually recommended that kids 1 to 2 years old drink whole milk.
Skimmed or 1% fat milk doesn't contain enough fat, so isn't recommended for children under 5.