Boiling cornstarch will cause it to break and go runny.
I have never made the sort of gravy used for biscuits, it sounds like from the above comments to similar to a white sauce or the base used to cheese sauce.
I always make a roux when I make gravy. The last couple years I have used rice flour instead of wheat flour or corn starch when making gravy. It takes longer to thicken, but works really well and the gravy does not break.