Mac, I always heard there was such a thing as a gas check seater. I never found out how it worked or what it looked like. The reason is, just putting the gas check on the base of a bullet made to take one, then sizing and lubing it seated the gas check just fine. In other words, it's not needed.
I'll add this in case someone else see's this, even though you probably already know it. You can't just put a gas check on just any cast bullet. The mold and bullet has to be made to take a gas check. There's a rebated heel on the very bottom of a bullet made to take a gas check.
Gas checks sold today are all made by Hornady. They're made to crimp on the rebated portion of the cast bullet. Take a close look at the gas check, a 10X magnifier will show you the inside edge at the top is a little thicker than the rest of the check. It is sized into the heel to dig into and hold the check on.
Lyman used to make a brass gas check that was a simple cup. They were notorious for falling off when fired. The Lyman checks sold today are made by Hornady, repackaged by Lyman.