Great question! Before answering straight away, let me first present some context about The Grob Opening.
The Grob Opening (1. g4), named after International Master Henri Grob (1904-1974), is one of the opening first moves for white with a very low draw rate (around 16%). Unfortunately for the fans of the Grob, after 1. g4 the black win rate is 45% so, statistically speaking, Grob’s Attack is not a very good opening choice for white. Still, 1. g4 remains a popular offbeat choice at the club level and many top GM’s have employed it in online blitz and bullet.
The greatest known defeat of a chess master with the Grob Opening occurred at the World Rapid Championship in St Petersburg Russia on 12/26/2018. GM Timur Gareyev (The Blindfold King) played 1. g4 in his fourth round defeat of the famous chess player and coach, GM Marat Makarov. The game finishes in a wild flourish of inaccurate play due to time trouble but white did secure a victory with The Grob in a prestigious over-the-board event so I believe this game to be the best example of a top grandmaster losing against Grob’s Attack.
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Result: 1 - 0