Refrigerator door seal is developing a gap towards the bottom. Not sure it is letting air through

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.

The refrigerator gasket is replaceable, but you'll need to find an exact replacement, possibly from the manufacturer.

Before doing that, make sure that the door hasn't warped, which would cause one of the corners to not close tightly. To straighten a warped door, loosen the screws that are beneath the current gasket (when you open the door, you can pull the gasket away to expose the screws). With the screws loosened, you can pull or push on the corners to get the door back to straight, and then re-tighten the screws.

If you decide to replace the gasket, then the process is to remove the existing gasket by loosening those same screws, and then installing the new gasket, starting at the top and working your way down and around the door. Designs may vary, but it should slide behind a metal or plastic clip/gap that is tightened via the screws. When you're done, verify the gasket is straight and tighten the screws in each corner. Check that the door isn't warped, and correct it now while it's still easy to fix, and then tighten the remaining screws.

When finished, double check for any gaps. I've seen suggestions to use a flashlight inside the refrigerator, close the door, and see if the light can be seen with the room dark. I've also seen suggestions from talcum power to a thin coat of petroleum jelly to lubricate the seal and/or prevent sticking. Follow your manufacturers suggestions or use your own best judgment there.