What are my options to treat buccal fat and double chin?

Thank you for posting your question here.I will start by stating that my answer is based on the limited information provided , for a more bespoke and detailed plan please contact your plastic surgeon.
Gaining weight after buccal fat removal is unlikely to replenish that fat pad, once it is removed the bulk of it has been removed and it is usually fruitless to go after it again, the weight gain most likely affected your subcutaneous fat ( the fat under your skin) . The best way to address that fat is weight loss.
That being said when it comes to the double chin , your surgeon will assess if it is secondary to fat or redundant skin , if it is secondary to fat then if he fat is above the platysma muscle then you can undergo liposuction again but if it is under the platysma muscle or if it is secondary to redundant skin then your surgeon will likely offer you a neck lift. There is also a chance of the double chin improving with weight loss especially if you are young and the skin still has the ability to contract.
I hope this helps, best of luck!

Without photos it is very difficult to discuss anything. However, generally speaking, reduction of fat in the neck is best controlled with an open approach like neck lift but, less accurate with liposuction. Removal of buccal fat is never a significant part of face shaping but face lift approach to reduce volume of fat in mid face, jawline, jowls and lateral neck can be used in selected patients. Hope this helps.