It was just the Seagulls' second goal in seven games and produced a welcome three points at the Amex where they had drawn their last six.
They controlled much of the game against a Watford side that suffered their fourth straight defeat and continued to slide down the table.
But Albion were made to wait for Gross' 64th-minute goal after spurning a number of chances in each half. Nevertheless, the midfielder's fourth goal of the season bagged the points and some early festive cheer by lifting them to 12th in the table.
Chris Hughton made three changes from the 0-0 draw with Burnley as Connor Goldson came in for the suspended Shane Duffy for his Premier League debut and Markus Suttner and Tomer Hemed also replaced Gaetan Bong and Glenn Murray.
Watford made four changes from the 4-1 home defeat to Huddersfield last week. There was a Premier League debut for Ben Watson, while Tom Cleverley, Roberto Pereyra and Andre Gray came in for Adrian Mariappa, Etienne Capoue, Abdoulaye Doucouré (suspended) and Troy Deeney (suspended).
The home side almost led on five minutes when Hemed raced away and twisted and turned in the box to beat his man, but fire wide across goal. The danger didn't end as Anthony Knockaert recycled the ball, but his low cross from the byline was pushed away by Heurelho Gomes.
Three minutes later, some great work by Bruno on the byline saw him flick the ball over his man, but Pascal Gross' pass to Davy Propper, saw the midfielder sky high and wide.
Moments later, Knockaert miscontroled in the box at the far post after Gomes failed to deal with Suttner's cross.
Watford grew into the game after weathering Albion's early storm, but without really troubling Maty Ryan. On the half-hour mark, Gomes made a good save to push behind Goldson's back-post header at point-blank range.
Referee Paul Tierney had a big call to make on the stroke of half-time as Hemed raced away on goal and last man Christian Kabasele chased him down and their legs tangled, with the striker going over just outside the box, but the official waved play on.
Albion started the second half with the same intensity as the first and a neat move between Hemed, Propper and Knockaert broke them into the opposition box, before Hemed eventually fired over.
Watford's best chance came on 55 minutes when Ryan was forced into a sharp save at the near post from Richarlison's snap-shot and the resulting corner was collected by the goalkeeper. At the other end, Gross then fired a long ranger across goal and just wide of the far post.
That built a period of sustained pressure which saw a Solly March cross evade Knockaert by inches and Gomes push away a dangerous cross, but Albion did lead on 64 minutes.
Hemed fed Gross and he turned on the edge of the box and fired right-footed through Gomes, who could well have done better with the hand he got on the shot.
And it should have been 2-0 four minutes later when Dunk rose highest from a corner, but powered his header just wide. Hemed was then fed by March and he was able to control and turn from just eight yards out, but somehow put the ball wide.
They were almost made to pay for missing those chances on 81 minutes as Richarlison met a left-wing cross but headed wide from just six yards, while at the other end, Knockaert fired into the side netting after Hemed's shot was blocked.
Murray came off the bench late on to make his 200th appearance for Albion, who held out the remaining minutes.
Albion: Ryan, Bruno, Goldson, Dunk, Suttner, Knockaert, Stephens, Proepper, March (Izquierdo 90), Gross (Kayal 82), Hemed (Murray 87). Unsued subs: Krul, Huenemeier, Baldock, Schelotto.
Watford: Gomes; Janmaat, Prodl, Kabasale, Holebas; Watson (Capoue 73), Cleverley; Carillo, Pereyra (Sinclair 70), Richarlison; Gray (Okaka 78). Unused subs: Bachmann, Mariappa, Wague, Folivi.
Referee: Paul Tierney.