The effect of weight gain on tummy tuck results depends
Ideally, do not undergo abdominoplasty unless your lifestyle and weight are stable and sustainable. If the weight gain is insignificant, 1-2% of body weight, then the results after abdominoplasty will not be impacted.
At some point, some weight gain will change the results of the abdominoplasty depending on why the surgery was done and what was done. If you had excessive fat that was removed, then weight gain may produce an increase of fat thickness but probably less than if you hadn't had...