Too, the cast bullet must be designed to accept a gas check. The bullet base is slightly less in diameter than the rest of the bullet. You can't realistically stick a gas check on a bullet with a full-caliber base (although I'm sure someone will prove me wrong - lots of "home-gardening" with reloaders).
Coupla kinds: Hornady's got one (I've never used it) that is similar in use/different in stickin' in on (?), but different than Lyman's which gets crimped onto the bullet base during the sizing operation.