Can a 25lb Weight Gain Stretch out my Muscle Repair?

Given your dimensions, it is unlikely that a 25 lb weight gain will affect the muscle repair. However, the overall results including the contouring will be adversely affected. With weight loss, this will be reversible. There may be a permanent slight decrease in the tightness of the skin; that is, it may be slightly more lax.

In the unfortunate event that a patient gains weight after a tummy tuck, the result often suffers.   Unfortunately, if you gain intra-abdominal fat and stretch the overlying muscle/fascia repair, it is difficult to correct these changes without revision surgery.  However, if your changes are confined to the subcutaneous fat, you may be able to improve your result with diet and exercise.     

I would recommend following up with your plastic surgeon to determine how your result has been affected.   It is very important to maintain a stable weight after a tummy tuck or you may end up unhappy with your longterm results.