You need to spell Gostelowe with an "e" on the end.
If you go here, it shows one of them, and there's a further link showing all 13:-
http://www.11thpa.org/unit_flag.html
I believe they were either catalogued, or found in a warehouse (can't recall offhand what Richardson's book says). Quite possibly he either made them, or ordered them to be made. Most of the ones I've seen appear to have gone to Pennsylvania units - Continentals, not militia though. Militia tended to have their own flags made locally by their wives.