Last updated on .From the section FA Cup
In the FA Cup, the World Cup winner scored two goals, including a wonderful flick finish.
After coming on in the second half, the ArgentinaMidfielder scored with a brilliant piece of skill and then tapped in a second.
The sides were level at the start of the game, with Chuba Akpom putting the side in front.
Adam Lallana scored from close range to make it a two goal game.
The hosts of last season's quarter-finals were thoroughly outplayed by the Seagulls.
Roberto de Zerbi took over from Graham Potter at the start of the season and has led the Seagulls into the next round of the cup.
The Seagulls showed their top-flight class by dominating Michael Carrick's Championship team, even though they lost on penalties in the Carabao Cup.
The south coast side plays easy-on-the-eye football and have many talented individuals who come together as one unit.
Gross opened the scoring after just seven minutes, slotting home after Solly March's effort was saved, but Akpom equalized shortly after, heading in for his 14th goal of the season.
Lallana pinched a goal from the impressive Kaoru Mitoma by nudging the ball over the line when the Japan forward's effort was already going in, as the Seagulls dominated after that.
They have a player who claimed the biggest prize in world football last month, and who is full of confidence.
He could have had a hat-trick in the 45 minutes he was on the pitch, showing his excellent form, technique and flair.
Mac Allister got his second by finishing off a slick team move after hitting the post with a curling free- kick.
With time running out, Undav came off the bench and drilled a low finish into the bottom of the net.