The Duddingston-attached player finished three shots clear of the field at Macdonald Spey Valley in the first 18-hole shoot-out for the Challenge Tour event at the same venue.
Gallagher, who turned pro after winning the Scottish Amateur Championship just under a decade ago, was bogey-free and birdied three of the last four holes as he carded a five-under-par 67.
“My game has been pretty good for a while and I played really solid,” he said. “I only missed one green all day and never had a dropped shot, so that was pleasing.
“The SSE Scottish Challenge is a big tournament that I have played a few times but not since 2013. My previous efforts have not been great, having missed the cut, but this effort has shown I can score around Spey Valley.
“I am looking forward to next week’s event and the success in the qualifier has also given me a great boost for this week’s PGA EuroPro Tour event at Montrose. Hopefully it is going to be a good couple of weeks.”
Glenbervie’s Fraser Moore secured the other spot up for grabs in next week’s event after winning a three-man play-off.