It was a bad start for Manchester United's new manager, as he lost his first game in charge of the club.
The home fans were desperate to see their team put a miserable end to last season behind them and they were stunned by Gross' first half double.
In the opening period, United were outclassed by Graham Potter's side, who went on to finish ninth in the league.
Although United did threaten a comeback when a scramble inside the visitors' box ended with Mac Allister turning the ball into his own net, it was not enough to stop the Seagulls.
This was the first time in nine previous league visits that the Seagulls had managed a draw.
There was a sense of optimism before the game as the sun shone and a new manager was in place.
Ten Hag has started to impose his personality on the club, to the extent of ousting some of the "prawn-sandwich brigade" from their corporate suites so his players could have their pre-match meal.
Many of the old issues still exist. The club shop had to be closed because of the protests.
The co-chairman would have heard the songs that called for his family to get out of the club while he was visiting.
There was no harmony on the field despite decent debuts from Christian and Lisandro.
Well drilled and fluid, BRIGHTON took advantage. United failed to impose themselves on the game until it was too late after Trossard volleyed into the side-netting.
For the opener, the run split the two men. The latter came across from the left to cover the cross and left a huge hole in the six-yard box for Gross to tap home.
The keeper turned into his path when Solly March cut inside and unleashed a low shot, even though he was in a similar position.
The half-time whistle was booed by the crowd. Within eight minutes of the restart,Ronaldo was brought in to lead a rescue mission.
The 37 year old had no effect. The confusion that led to United's goal was caused by Diogo Dalot. By the end, it was the Seagulls who were more likely to score.
Potter's influence at the Seagulls should not be underestimated.
He has lost two players from the team that beat United. The director of football has left. The summer signings didn't start the match.
They moved the ball with a purpose that wasn't related to United.
Gross and Trossard were both superb and, even without Bissouma,Brighton controlled the middle of the field to prevent United from turning their industry into something more tangible.
Can we play you every week?