I'm 5'7 158lbs Going for BBL, Can I Gain Weight? (photo)

Hello dear!
Thanks for the question and provided information as well.
Based on your pictures, you are a great candidate for liposuction with BBL, and I think you'll get very good results. 
Liposuction allows your plastic surgeon to transform your entire shape. You can improve your abdomen, thighs, hips, back and flanks, and of course your buttock, to give you a nice heart shape.
In general, you need to be physically healthy and at a stable weight, have realistic expectations, be a non-smoker and your BMI needs to be 30 or less.
l recommend you to make an appointment with a board certified plastic surgeon to talk about your goals and anything you need to know.
Good luck :)

Thank you for your pictures.  You should follow your PS's advice on this matter.  I do not recommend that for my patients.

Dr. ES

Based upon your height and weight I don't think you need to gain any weight for the BBL.  You need to consult with a Board Certified PS with extensive experience performing the BBL. You need to view many pre-op and post-op photos to get a sense of possile results. Good luck!