What is a Gamma Seal? It is a screw on lid that one puts onto a plastic food grade bucket for long term food storage. NOTE: Make sure you really want the lid on your bucket because once you hammer the ring onto the bucket, use a rubber mallet, it's there for good! You'll have to destroy the bucket to get the ring off.
WalMart has these in stock in various stores around the US for $4.38 each. If your WalMart isn't carrying them tell them you want them and here is the sku #076939714027