That test might help my research. I do not know one of my great grandfathers, or 2 of my 2g grandfathers due to illegitimacy.
Are there any direct male line descendants still alive who might agree to take the test? The nature of a Y-DNA test is that there must not be any female between your great grandfather (or 2g grandfathers) and the person being tested. This is because the Y chromosome is only passed from father to son. Having a Y chromosome is what makes someone male.