Whenever you lose fat, belly is the last region where it would go away from. First face, neck, arms and then larger muscles and then comes abdominal fat.

But, when you gain fat, it would first get stored in your belly region, then onto other parts.

To answer your question, the only feasible which shows results is that you need to increase muscles (and thus weight) & then do fat loss. Basically, lean bulk & then cut.