Should I gain weight before my BBL? I am 5’2 and 140lbs. (Photos)

I would not recommend you gain weight, you seem to be a good candidate for liposuction with bbl.always see a board certified PS for a detailed consultation.

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).