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

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

          I do not think that gaining weight will be beneficial.   You can get a very dramatic improvement.   I typically transfer between 1200 and 1500 cc per buttock.  I perform about 500 Brazilian buttlifts each year, and this procedure is the most surgeon dependent plastic surgery procedure.  You do not want to have to do the procedure again.  Find a plastic surgeon with ELITE credentials who performs hundreds of liposuction procedures and Brazilian buttlifts each year.  Then look at the plastic surgeon's website before and after photo galleries to get a sense of who can deliver the results.

Kenneth Hughes, MD Los Angeles, CA

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 :)