Albion were superb throughout and took a deserved 15th-minute lead courtesy of Moises Caicedo’s first goal for the club.
Gross’ initial attempt was blocked by Alex Telles before Ecuador midfielder Caicedo drilled into the bottom left corner from distance.
Albion should have doubled their lead three minutes before the break.
Raphael Varane made a mess of Robert Sanchez’s long kick forward but former United forward Danny Welbeck lobbed over with only David De Gea to beat.
Brighton did increase their lead early in the second period.
Marc Cucurella joined Caicedo in claiming his first Albion goal, sweeping into the roof of the net after Leandro Trossard pulled the ball back from the left.
United were in danger of being humiliated and fell 4-0 behind after conceding quickfire goals to Gross and Trossard.
German midfielder Gross slipped the ball beyond De Gea from close range, with Trossard again providing the assist after Cucurella had collected a long pass from goalkeeper Sanchez.
Trossard then bundled home from close range after Diogo Dalot cleared Welbeck’s initial dinked effort off the line.
VAR checked for possible handball but the goal was awarded, with United fans chanting “you’re not fit to wear the shirt”.
Brighton fans menwhile finished off singing “we want five!”
It was by far and away Brighton’s best home performance of the season and a proud moment for head coach Graham Potter.
Here’s Sonny Turner with the Brighton player ratings...
Albion should have doubled their lead three minutes before the break.
5. Leandro Trossard- 8
A threat throughout the game. Maintained composed to pick the right ball, pulling it back to Cucurella who fired in the Brighton second. Showed great determination to bundle the Brighton fourth over the line, putting Brighton in dreamland. Photo: Mike Hewitt
6. Solly March- 8
Started the game very impressively, and was a danger down the right had side all game. His creativity and precise passing allowed Brighton to trouble United and put them on the back foot early in the game. Photo: Bryn Lennon
7. Moises Caicedo - 9
Great goal to give Brighton the lead, driving the ball low and hard into the left corner from the edge of the box. Made passing look easy today, and controlled the midfield, which gave the Seagulls the platform to hurt United, whilst also preventing the visitors from creating anything of their own. Photo: Manchester United
8. Pascal Gross- 8
Had linked up well with March before the goal, and looked dangerous. Found himself free in the Brighton area, skipping past one challenge and lifting over De Gea’s outstretched leg to give Brighton a third with a very well taken goal. Photo: Mike Hewitt