For a not so distorted, and want to bring the curves inside, you might want to try Filters > Distorts > Lens Distortion...
The thing is we don't see your image, thus we can only speculate, post a sample of your image or a reduction, and you will have real solutions
The thing is we don't see your image, thus we can only speculate, post a sample of your image or a reduction, and you will have real solutions
Patrice