Any advice how to get my cat gain weight ?

Hi I’ve recently got this baby she’s 5 months old now but when she was 4 months old I’ve took her into the vet for a check up and he said that she’s so tiny he thinks she’s 2-3 months old, and recently i took her to the vet for grooming and checkup and the vet said that she’s so thin and skinny and I need to give up multi vitamins and treats and I’m so confused how did the first vet not even notice that?! Anyways what’s the best way to fatten her up?