My dog is very small she is 28 lbs and she is 6 months old I have a pitbull I wanted to know what could I feed her so that she could gain weight or is there any weight gain supplements that she could take her ribs are visible she looks sick her doctor said just keep food her her bowl but no improvement and she is well no sickness

Does she eat normally? Any vomiting?
I am assuming she has had stool checks for parasites, blood work to rule out liver or kidney problems?
Is she eating a good balanced puppy food?
Some dogs are just "poor dooers" but I wouldn't count on that until I had ruled out everything else that could cause failure to gain weight. You may want to consider a second opinion.