If I start to gain weight back will it go to my butt or will those fat cells no longer exist?

Hello and thanks for your question!  The fat that was transferred to your buttocks will behave like it did when it was in the part of the body from which it was taken.  This was probably from your abdomen.  If you tended to gain or lose weight in your abdomen, your butt will now react that way, rather than the way your butt reacted prior to the surgery.
Hope that helps!

Patients will commonly wonder what the effects of liposuction will be on their weight and more specifically their ability to gain or lose weight in treated and untreated areas. 
After puberty, an individual will have the same population of fat cells for the remainder of their lives (barring an intervention). Any changes in weight will manifest not as more or less fat cells but rather swelling or shrinking of those very same fat cells.
Liposuction destroys or reduces the population of fat cells in a treated area. These cells once gone are gone for good. The treated areas now harbor a reduced population of cells and consequentially have a reduced potential for weight gain or weight loss. However, a reduced potential does not make changes in these areas absolutely impossible. The remaining cells can still swell with weight gain potentially compromising a cosmetic result.
The final component is the least understood. This is that of weight/fat distribution. It remains unclear how the body directs particular populations to change. Patients will often lament that attempts at weight loss are frustrated by losses in areas not targeted and weight gain by contrast goes directly to perceived problem areas. It is impossible to predict how a patient will respond to weight gain. While we know that treated areas must theoretically have a lower potential for growth by virtue of reduced adipocyte populations, this does not necessarily always hold true with some patients always demonstrating the effects of weight gain in the same predictable areas. Similarly, post operative weight gain is not always manifested in untreated areas (i.e. your stated concerns of ballooning in the face/upper back/calves).
The best advice one can give is to reach a stable healthy weight pre-operatively and try to maintain post-operatively.
As always, discuss your concerns with a board certified plastic surgeon (ABPS).