How do I get me pitbull to gain weight?

If a vet said it was under weight, he should have given you recommendations. Otherwise, read through the sticky at https://www.askmehelpdesk.com/dogs/information-articles-our-dogs-expert-labman-53153.html#post254171 If you have more questions after reading it, post back. It is exceedingly rare for a healthy dog not to eat all it needs. It is much more common for its owner to think it needs to eat more. Tempting a dog with rich food is nearly always the wrong thing to do.