Replacing the freezer door gasket on a Whirlpool french door refrigerator (model GX5FHDXVQ02). Age caused the gasket to become over-compressed which resulted in frost build-up around the interior edges of the freezer door. This procedure should be similar for most types of refrigerator and freezer door gaskets. See the links below for the gasket I used:
White Freezer Door Gasket (W10443319):
https://www.ebay.com/itm/392268537725
-OR-
https://www.searspartsdirect.com/product/4e9gvjpyua-0046-106
Note: If you don't care about color, sometimes it's quite a bit cheaper to go with the gray gasket (W10571967).