Do I need to gain weight for a BBL?

With a Brazilian Butt Lift, the amount of volume that can be added to the buttocks depends on the amount of fat you have in your abdomen, flanks, bra rolls and thighs to be removed via liposuction, and then transferred to your butt.  Even with minimal fat to transfer, a BBL is oftentimes possible as long as the client is looking for natural, athletic looking bottoms as opposed to a round, voluptuous one.  There are many different butt shapes, and if done correctly, a small amount of fat can add a subtle lift. Sometimes a small modification can make a big difference.
For my thinner clients looking to obtain a fuller derriere than what a BBL would provide, I typically recommend using fillers to enhance their butt size.  The use of an injectable poly-L-lactic acid helps stimulate the body's natural collagen production, which adds volume to the area. 
Always choose a board certified plastic surgeon with experience in these procedures.  Best wishes!

Hello dear!!
Thanks for the question and provided information as well. 
Unfortunately, without pictures is very difficult to give you a proper answer.
I recommend you to make an appointment with a board certified plastic surgeon for a good evaluation and to talk about your goals.
Good luck :)