Hi there-
Gaining weight before your Brazilian Butt Lift makes sense if there is a reasonable likelihood that by doing so, you will be able to achieve a shape and proportion that you are happy enough with that you will want to maintain it over time, or if your experienced BBL surgeon believes that by gaining weight, you are increasing the likelihood of success.
While I cannot speak for other surgeons, in my own experience I have found, and believe, that by enlarging the available fat cells through reasonable weight gain, harvest of the maximum number of cells with maximum survival and the greatest margin for error is achieved. You can do better with less risk.
While the absolute number of cells is not altered by the weight gain, the risk to benefit ratio is favorably shifted in our favor.