Former Liverpool defender Hyypia only took over as Albion boss in the summer but the Seagulls have gone from promotion hopefuls to relegation candidates in the space of four months.
The under-fire Finn wrote in his programme notes that it was “time to deliver”, but his rallying cry fell on deaf ears as his side slipped to a third straight defeat, with a wretched run of one win in 17 games leaving them floundering in the drop zone.
Brighton’s attack was led by former England striker Darren Bent, but it was former Halifax striker Gregory who settled the match with his fourth goal of the season.
His 15th-minute strike provided a much-needed boost to Hyypia’s opposite number Ian Holloway, whose side arrived on the south coast on the back of a similarly miserable run of one win in 15.
Scott Malone’s low cross was only cleared to the edge of the area, from where Gregory played in Magaye Gueye and continued his run into the box.
French forward Gueye went for goal himself but his mishit shot became the perfect return pass to Gregory who lifted over Brighton keeper David Stockdale and into the net.
Brighton huffed and puffed in search of an equaliser but, with Bent on his own up front, they lacked any sort of cutting edge, Anfield loanee Joao Teixeira was their chief threat, but he was denied twice by Lions goalkeeper David Forde, once from long range and then smartly at his near post before the interval.
After the break Teixeira slid in Bent, but the on-loan Aston Villa frontman could only hit the side-netting.
Brighton’s best chance fell to winger Solly March, who met a cross by left-back Adam Chicksen 15 yards out but somehow missed the target.
Teixeira tried his luck again after he was sent clear by Elliott Bennett but this time Portuguese midfielder dragged his shot wide, while Craig Gardner’s header back across goal was hacked clear by Mark Beevers.
Chants of ‘we want Sami out’ could be heard from the home crowd during the second half, although a mindless section of Millwall fans did their best to tarnish their side’s win by breaking through the stewards in an attempt to rush the Brighton end.
Nevertheless, on the pitch it was relief for Holloway while time could be rapidly running out for Hyypia.