Should I gain weight for BBL? I want 1600cc in each cheek but not sure if I have enough fat for it. I want dramatic results!

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. An exam would provide a better way to evaluate you for the procedure and determine the amount of fat available for transfer. Most patients get an immense benefit with maximal liposuction of the abdomen, sides, and back to obtain the smallest waist possible and then maximal fat transfer to the buttocks and hips. 

Thanks for your question. When planning a Brazilian butt lift, patients can look forward to a double benefit: reducing areas of unwanted fat while gaining a perkier, rounded posterior.  Each procedure must be planned according to the individual needs of patients, harvesting fat from target areas of the torso, thighs or knees and transferring it to create smooth, attractive contours optimized for each person. The amount of fat transferred depends on the quantity available and the extent of buttock augmentation each patient desires.  Sufficient body fat for a buttock augmentation is very patient-specific, depending on an individual’s body shape and proportions.If you put on weight before surgery, there is more to transfer.  But I do not generally recommend this because we augment the buttock during surgery for optimal shape, and if you then lose the weight post operatively you will loose it disproportionately from your augmented buttock and the results could suffer.  We recommend patients to keep their normal weight and body form prior to surgery, and then we are working from a stable situation to create the changes we are working for. 
That said, you definitely have to choose your surgeon correctly and make sure that he/she has the training and experience to meet your goals with absolute safety.  Best is a Board Certified Plastic Surgeon with specialized expertise in this procedure.