The Italian second seed and four-time major winner was in a commanding position, leading 6-2 6-1, when Gaston withdrew with an abdominal injury on Tuesday night.
It shocked the packed Rod Laver Arena crowd but came as no surprise to Sinner, who could sense his opponent struggling from midway through the first set.
"I saw that he was not serving with a very high pace, especially in the second set," Sinner said post-match.
"It's not the way you want to win the match, but he's such a talented player.
"He has incredible touch, moving very, very well, so I knew starting from the beginning that I had to play a very high level of tennis."
Sinner hit the ground running in his bid to match Novak Djokovic's feat of winning three straight Open crowns - which the super Serb has done twice.
The 24-year-old sent down six aces to none and won 86 per cent of points on his first serve, while denying Gaston three break points.
Importantly, Sinner was able to conserve energy for later in the tournament, with the first-round encounter lasting just 68 minutes.
"(I was) trying to be as aggressive as possible, which I've done, so I'm very happy," Sinner said.
"I'm very happy to be back here. It's a very special place for me."
Gaston also battled injuries last year, withdrawing from the Australian and French Opens with similar abdominal issues in slightly different areas, and succumbing to an achilles issue in Metz in November.
The 25-year-old, who won the 2018 Open boys' doubles title with Clement Tabur, was in tears on his courtside bench after conceding defeat to Sinner.
"To retire against a player like Jannik on such a big court, big event, it's always sad," Gaston told AAP.
"It was a good first set, but when you play Jannik at full capacity it's tough, and if you are a little bit less, it's impossible.
"Out of respect for him, I cannot continue the game. I just had to stop.
"It was a tough decision, of course, but I need to keep my body healthy."
Sinner consoled his opponent, who was far from disgraced and entertained the crowd with a smattering of superb winners.
The reigning champion showed no major signs of rust in his first competitive match since beating world No.1 Carlos Alcaraz at the ATP Finals in Turin in November.
He took confidence from a hard-fought exhibition victory over Felix Auger-Aliassime four days out from the start of his Open title defence.
"I felt very well prepared. We worked a lot physically, also on the court too," Sinner said.
"I had one great match with Felix, even though it's an exhibition, but we went full.
"But the official matches are always very different, so I'm very happy with how I started off today.
"Of course a bit of tension, but now it's time also to enjoy. All the hard work is done and we practise for moments like this, so I'm very happy to be back here."
Sinner next faces a clash with Australian wildcard James Duckworth, who reached the second round in vastly different fashion, advancing through a gruelling five-set encounter with Croatia's Dino Prizmic, 7-6 (7-4) 3-6 1-6 7-5 6-3.
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