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...
1. Robert Sanchez - 8
Pulled off an acrobatic save from a close range Edinson Cavani header to keep his clean sheet intact, followed by a good stop against Bruno Fernandes from just outside the area. Another good stop at his near post denied Harry Maguire’s headed effort. Great pass in build up for Brighton's third
Photo: GLYN KIRK
2. Marc Cucurella - 10
A fantastic performance. Showed great work rate, tenacity, and an insatiable hunger to win the ball. An early second half goal fired into the top corner was reward for a great all round performance.
Photo: Bryn Lennon
3. Lewis Dunk - 8
Really composed at the back all game, unfazed by the presence of Ronaldo. Marshalled the defence, and proved too much for United to trouble.
Photo: GLYN KIRK
4. Yves Bissouma- 7
Showed his usual composure and skill on the ball, and was not the least bit worried about pressure on the ball. Passed well and controlled the midfield alongside Caicedo.
Photo: Mike Hewitt