It's a mild blasphemy. Literally "(May) God blind me." It's now old-fashioned and would be used mostly by the older generation. "Blimey" (without "Gawd") is still popular across the generations, though less so among young people. I think it's common also in Australia, though others would have to confirm.

It is an interjection , exclamation, or intensifier that roughly means My God "cor blimey " and just "blimey" are also used "Gawd blimey he's in trouble again!" Gawd blimey she's late."