United barely registered an attempt all evening and were deservedly on the end of a seventh league defeat of the season, meaning that they have not yet secured second place with two games to play.
For Brighton, though, just a second win over United in 19 meetings ensures that they are safe of the drop with two games still left to play.
The early signs were promising for United, as Marcus Rashford won his side a free kick in a promising position and sent it into a dangerous area for Marouane Fellaini to convert, only for the Belgian midfielder to rightly be flagged for offside.
From that point on the Red Devils barely put together a fluid attacking move in the first half, as a side showing six changes from last time out struggled to keep their foot on the ball.
Brighton arguably did enough to take a lead into the half-time interval, with both Gross and Jose Izquierdo being denied with long-range efforts, while Dale Stephens dragged one wide from a similar range.
The Seagulls' best effort came via returning striker Glenn Murray, whose half-volley was dipping towards the bottom corner if not for the intervention of De Gea to get across.
Visiting boss Jose Mourinho was struggling to hide his frustration on the touchline, his mood made all the worse when his side finally got in behind through Rashford, who delayed a simple cutback to Anthony Martial and saw the chance go begging.
After getting in their first on-target attempt of the contest early in the second half through Paul Pogba, United were soon back to looking their cagey selves.
With less than an hour played Brighton finally managed to find a way through, doing so via a Gross header from eight yards that Marcos Rojo very nearly kept out on the line on a rare start for the Argentine - goal-line technology coming to the Seagulls' aid.
Anthony Knockaert then had a penalty shout turned down after appearing to be clipped by Martial, and Izquierdo - who set up the winning goal - blasted wide from a good position.
For the first time all evening the visitors were finally enjoying some sustained pressure, coming close through Rashford's long-range drive and Lingard's curler, both of which Mathew Ryan was equal to.
The final chance of the match fell to Lingard, who was unable to keep his shot on target after latching on to Rashford's backheeled pass to spark a mini pitch invasion at the Amex Stadium.
BRIGHTON & HOVE ALBION (4-4-1-1): Ryan; Bruno, Dunk, Duffy, Bong; Knockaert, Stephens, Propper, Izquierdo (March 88'); Gross (Kayal 84'); Murray (Ulloa 93')
MANCHESTER UNITED (4-3-3): De Gea; Darmian (Shaw 68'), Smalling, Rojo (McTominay 76'), Young; Matic, Fellaini (Lingard 68'), Pogba; Mata, Rashford, Martial