What does that foreshadow? You never actually see anyone getting hanged even if you screw up. You can get strangled or have your heart ripped out, but I've never gotten hanged. Actually, the old man getting strangled by the snake makes more sense since Gabriel can die in that way, but I think the old man is supposed to be Gunter not Gabriel.
Yes, the hanging man is Gunther. The implication is that following the loss of the talisman, he completed his diary and sent it back to Germany to confess to his failures and then hung himself. In Gabriel's dream, he imagines himself being hung as it represents the Ritter curse being passed down through the generations (it seems likely that Wolfgang would see himself as the hanged man, as would have Gabriel's father and grandfather etc.)