In his first official match since beating Carlos Alcaraz for the ATP Finals crown, the Italian world number two was in ominous form on the Rod Laver Arena court he has ruled for two years before the match was cut short.
After losing the second set, Gaston gestured that he could no longer continue and retired to his seat to cry under a towel.
Sinner said he was surprised by the retirement but could see that Gaston was not at his best.
Sinner will continue his bid for a third successive Australian Open crown against the winner of James Duckworth and Dino Prizmic.