Is potato flour or starch a good GF option to thicken need bourguignon? I can't use dairy either.

I like to use a cornstarch slurry. It gives the stew a nice glossy sheen and a little goes a long way.

Potato flour has a potato flavor, is courser and harder to find than potato starch. So, I would use potato starch, tapioca starch or cornstarch. I don't usually thicken my stews at all, but I haven't made beef bourginon in a long time.