IF your diet is in check NOW then give it a while, 6 months or more to gain some weight naturally because you are under weight now and it's due to not eating right.
Most of us have been there/where you are and we where 100% it was because we where hard gainers. No it was diet. 99.99% of us it's always diet no matter what we think. Just like obese people who say it's a thyroid problem. LOL
I bet 10 years from now you will look back and say/think you where a dumb kid.

Anyone who doesn't is still stupid and hasn't learned anything.