Nearly any chert will work ok, but some are much better than others. Generally speaking dark flint makes better gunflints than lighter colored flint/chert.
I had to make a gunflint at an event from a red chert that is very good. I kept a coupla pieces and use them for hunting.
There is a southern blue flint that is VERY good.
Texas has a gray flint that is good too.
White chert, found here in the Ozarks, throws a surprising pile of sparks, but is pretty brittle and only lasts a few shots.
Run a search on flintknapping to find articles on knapping arrowheads. If you visit a knap in, don't tell them you want to make gunflints. For some reason, point knappers look down on anyone who wants to make gunflints and won't teach you anything.
J.D.