Albion had not found the net from open play for more than 10-and-a-half hours when German midfielder Gross cut on to his right foot and let fly with a 64th-minute drive which flew under Watford goalkeeper Heurelho Gomes.
The well-earned win was much needed for Seagulls manager Chris Hughton, ending a run of three points from a possible 27 while inflicting a fourth successive loss on the Hornets and putting a little more pressure on their boss Marco Silva.
The margin of success should have been more resounding as Brighton dominated for large periods of the contest and Lewis Dunk, Tomer Hemed and Anthony Knockaert each squandered excellent opportunities to make it 2-0.
With their strong start to the season having turned a little sour in recent weeks, Watford appeared devoid of confidence and direction for much of the match, although Richarlison and Stefano Okaka wasted superb late openings to snatch a point.
It was the first top-flight meeting between the two clubs since a 1-1 draw at the Goldstone Ground on New Year's Day 1983 and, given the recent struggles of both teams, the cautious start was unsurprising.
Albion, whose last victory came at bottom club Swansea on November 4, had the better of the early exchanges.
Recalled Israel striker Hemed, starting in place of Glenn Murray who missed a costly penalty during last weekend's draw with Burnley, flashed a shot across goal inside five minutes before Knockaert's follow-up was turned away by Hornets goalkeeper Gomes.
Holland international Davy Propper then skied an effort from the edge of the box after good work down the right from enterprising Seagulls captain Bruno.
Cleverley was back in the the Hornets team following a one-match suspension and he was left in a heap in the centre of the pitch with 16 minutes played following a studs-up challenge from Albion midfielder Propper, which resulted in a free-kick but no card from referee Paul Tierney.
The defender, who underwent heart surgery in March, almost marked the occasion with a goal, sneaking in unmarked at the back post from Pascal Gross' left-wing corner and seeing his header tipped wide by Gomes.
The hosts then felt aggrieved moments before the interval as Hemed raced towards goal and went down under a clumsy challenge from Christian Kabasele which went unpunished.
Watford had lost just one of their previous 11 clashes with the Seagulls and they registered their first meaningful attempt on goal 11 minutes into the second period.
Richarlison exchanged passes with Pereyra on the left before the Brazilian winger's effort was pushed around the left post by Albion goalkeeper Mathew Ryan.
Brighton last scored from open play on November 20 but they duly provided the breakthrough the game needed with 64 minutes on the clock.
Summer signing Gross claimed it, unleashing a powerful shot from 18 yards which Gomes really should have saved.
Brighton were not resting on their laurels and defender Dunk headed agonisingly wide from a Gross corner, before Hemed somehow steered his shot on the turn past the left post having been picked out by Solly March.
Richarlison, who had been unusually quiet for the Hornets, then almost silenced the jubilant home support with nine minutes to go when he somehow nodded wide from close range following Jose Holebas' deep delivery into the box, before substitute Okaka failed to capitalise on Ryan's dropped catch as the hosts hung on.
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