How do I fix a gap in my refrigerator door gasket seal?
Measure the space between the door and refrigerator on all sides/corners to see if something is out of alignment. One of the steps in replacing most gaskets is to fix the alignment of the door before tightening the screws. To fix the alignment, you would loosen the gasket screws, push or pull the top and bottom corners of the door until it's properly aligned, and retighten the gasket screws.

See these instructions on the steps to replace a gasket to get a better idea of the steps involved.