Keres-Arlamowski 1950, comes very close
[FEN ""]
[Event "Szczawno Zdroj"]
[Date "1950"]
[Result "1-0"]
[White "Paul Keres"]
[Black "Edward Arlamowski"]
1. e4 c6 2.Nc3 d5 3.Nf3 dxe4 4.Nxe4 Nf6 5.Qe2 Nbd7 6.Nd6# 1-0
I heard Mr Arlamovski was an IM at the time - but given Keres' level, and the fact that there was much fewer IM's back then - probably puts the game into the GM level category.
