Do I need to gain weight for a BBL?

I generally do not believe weight gain pre-operatively is a good idea. This is true especially considering the fact that post-operatively some patients will struggle with losing the weight again. The optimum surgical candidate has reached a healthy and stable weight. 
This issue often comes up when discussing BBL and fat transfer. For these patients, adequate donor sites for harvest are necessary. The question of "adequate" will vary from one patient to the next. Your board certified plastic surgeon is uniquely suited to asses whether you have the reserves to meet your goals.
If you have reached a healthy stable weight (preferably with a BMI less than 30), the focus should be on maintaining. I typically do not recommend gaining/losing weight for the sake of surgery alone (weight loss for the sake of your health is a different story). It is important to remember that weight changes post procedure can potentially compromise the results of your buttock augmentation (as transferred fat can change with weight just as it could prior). If you plan on losing weight, I would do so before surgery and plan on maintaining afterwards. Weight gain should be discouraged unless you plan to maintain post operatively (or else the fat gained and transferred is typically the first to go post op).
Ultimately, decisions are based upon a patient's particular expectations. Some patients only want a subtle change and may be willing to accept a less significant result. But the rule still applies you have to have the fat to transfer the fat.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).

While gaining weight before a procedure is not recommended, If you were to do this then it is important that you maintain the weight. If you were to lose the weight after then the results of the procedure would shrink as well. 

Hello cherrydoll, 
I appreciate your questions! Many of my patients who desire a fuller, beautifully rounded backside opt for Brazilian Butt Lift (BBL) surgery. One of the main considerations when evaluating a patients’ candidacy for Brazilian Butt Lift surgery is whether they have enough fat stored in their body so the surgeon can harvest enough fat cells to use during the fat transfer phase. For patients with insufficient fat deposits, the doctor may advice weight gain before the BBL to qualify for the procedure. Set up an in-person evaluation with an experienced plastic surgeon to assess your BBL’s candidacy and discuss your specific needs and aesthetic expectations. Good luck and may God bless you! Dr. De La Cruz.