I was asked to gain weight for BBL. How can I make sure I am gaining subcutaneous rather visceral fat?

Your goals as well as your selection of your surgeon are important.  Gaining weight is often counterproductive. If you gain weight that is fat, some of the fat will likely be associated with the buttocks.  In almost every case, I do not recommend weight gain prior to a BBL. I think it is better to maintain your stable weight.

Dear Anika3555,
it is hard to tell for sure without an examination. Generally speaking, surgeons don't recommend their patient to gain weight prior to surgery because it will affect the results once s+you start losing it after surgery. If you are considering a surgery, I would suggest you to consult a board-certified plastic surgeon. Only after a thorough examination you will get more information and recommendations.
Daniel Barrett, MD
Certified, American Board of Plastic Surgery
Member, American Society of Plastic Surgery
Member, American Society of Aesthetic Plastic Surgery

You cannot change your own fat distribution. It is not related to the type of food you eat. It is determined by genetics and to some degree hormone balance.
I do not recommend anyone gain weight for fat transfer procedures.
Often the end result is looking overweight with increased visceral fat and a full looking face.
Is it better to work with smaller volumes of fat or not have a fat transfer procedure and stay lean.
If anyone loses weight after the fat transfer procedure the whole purpose of weight gain becomes pointless regardless.
Different plastic surgeons have different views on this topic. 
Without pictures we can’t make an assessment.
The answer to your question is you cannot control gaining visceral fat and weight gain will always increase this real fat to some degree.
For those who have a higher percentage or excess visceral fat weight gain is detrimental and will be counter productive.
Best, 
Mats Hagstrom MD

You do not need to gain more weight for a BBL. High defintion VASER liposuction has changed fat harvesting and fat grafting techniques. This way patients with smaller builds, like yourself, can still have amazing BBL results. If you are a good candidate for a Brazilian Butt Lift (BBL), the optimal outcomes are achieved with: 1) use of VASER liposuction to harvest the fat, 2) a closed looped system such as Puregraft to process the fat, and 3) strategic micro-packet transplant technique. These three processes have demonstrated over 95% fat graft take. This means that your results will be stable as long as you maintain your weight. As an official VASER trainer, I am routinely referred unhappy BBL patients. One of the most common reasons for disgruntled BBL patients is that patients are not able to accurately communicate their desired buttocks goals to their surgeons.  A buttocks assessment tool can be an invaluable tool to communicate to your surgeon the exact buttocks size and shape you are desiring. A Skype or FaceTime consultation with a BBL specialist can help you get an accurate recommendation and pricing. 
Respectfully,
Arian Mowlavi MD,FACS
Realself 100 Hall of Fame Inductee