they may have been ethnically Polish, they had no Polish citizenship.
That's very wrong. You should read the Versailles treaties before posting such nonsense. The Second Republic had to promise the Allies that in return for its rebirth as a nation, that it would not discriminate against people for arbitrary reasons, especially minorities. By default, they became citizens, not stateless people, even if living abroad.
My grandfather was born in 1908.
Your claim depends on the old Hapsburg laws in conjunction with the Versailles treaties. The present government is quite hostile to claims like this, regardless of the old treaty obligations. Other than possibly ethnic Germans living outside the borders of the Third Republic at its creation, those obligations haven't changed over time. A similar case is making its way up the courts. You may want to wait and see how it is decided before filing your claim. To file, you need to find your grandfather's birth records, his parents' marriage record, and either your great-grandfather's birth record or his military service records, (which are very hard to find). The complicating factor is that your grandfather was born a dual citizen of the U.S. and, what is usually simply called Galicia. Normally, countries ignore competing claims on its citizens from other states, and recognize them only as their own. The present government has a policy of recognizing competing claims over its own. They have lost this argument years ago in an international court, but continue do it defying international law. For any claim like this, a good lawyer is required. Do not file this claim without a lawyer. Again, I suggest that you wait.
There is another blog about this, but I won't post the link.
Ignore the trolls who haven't been through this process themselves.