The Pilgrims' mission to avoid a relegation scrap have been boosted in recent weeks with the sudden resurgence of the in-form striker, on-loan from Blackburn Rovers, and his double certainly helped their cause.
Gallagher, who had scored the winner for the Pilgrims at Wolves on Saturday, opened up the scoring for Argyle towards the end of the first half after referee Nigel Miller awarded a penalty.
Winger Alan Judge was deemed to have been fouled by Hornets defender Gavin Hoyte inside the left-hand corner of the penalty box and Gallagher subsequently rocketed the ball into the roof of the net.
It was no more than Argyle had deserved after teenage striker Ashley Barnes, making his full debut, twice hit the bar with the game goalless, first with a powerful shot that Watford goalkeeper Scott Loach got a hand to, then with a looping header.
Watford manager Brendan Rodgers' decision to introduce Tommy Smith at the beginning of the second-half soon paid dividends for the visitors towards the hour-mark.
A defensive error in the Argyle back-line was quickly punished by the striker, who rounded goalkeeper Romain Larrieu to slot home.
Plymouth's defence soon rallied following the visitor's equaliser and they continued to absorb the pressure that was being thrown at them.
As Watford continued moving forward in search of a match-winner it was Gallagher who struck again on the counter-attack to reclaim Argyle's lead.