The full-back took advantage of Danny Care's chip, and a mistake from his new England colleague Charlie Sharples, to touch down in the 73rd minute in the Pool Six clash on Saturday.
Gloucester had proved a tough nut to crack up until that point, with the Cherry and Whites cancelling out Matt Hopper's opening try with an effort from James Simpson-Daniel.
Harlequins trailed 11-10 at the interval and they were 14-13 down when Chris Robshaw, one of the leading candidates for the England captaincy, three times declined to take the points as Gloucester were penalised at successive five-metre scrums.
But Harlequins conjured up one last opportunity which Brown snapped up, and a victory in Connacht next week should be enough to secure a quarter-final place, even if Toulouse top the group.
The French champions were made to battle all the way by a spirited Connacht side but eventually sealed a bonus-point 24-3 victory.
It may have been Connacht's 14th successive defeat but they will take a lot of encouragement from this display at the Stade Ernest Wallon.
Niall O'Connor kicked the visitors' only points while Louis Picamoles, Maxime Medard and Timoci Matanavou (2) all crossed to score tries for Toulouse.
Meanwhile Munster were far from convincing but their 26-10 victory over Castres in Pool One confirmed them as the first qualifiers for the quarter-finals.
Johne Murphy's eighth-minute try at Thomond Park set the leaders on their way to a 16-7 half-time lead, with Yannick Caballero hitting back for the French outfit.
Although Castres had nothing but pride to play for, their understrength side ensured a frustrating second half for the home faithful until Wian du Preez touched down with 12 minutes to go.
Unforced errors and missed scoring chances made for a poor display from Munster, who will need to up their game at Northampton in round six as they chase a home draw in the last eight.
The Saints produced a superb second-half fightback to end the Scarlets' quarter-final hopes with a thrilling 29-17 victory at Parc y Scarlets.
The Welsh side needed to win to stay in the hunt for a best runners-up spot and had the better of the first half as a try from wing Vili Iongi plus nine points from fly-half Rhys Priestland helped them to a 14-6 interval lead.
But Northampton's renowned forward power changed the complexion of the game after the break as tries from Soane Tonga'uiha and Ben Foden, supplemented by Stephen Myler's 17-point haul, saw them secure a fifth win in a row in all competitions.
In Pool Two, Cardiff Blues marked their 100th match in the competition with a tight 22-15 victory at London Irish to move a giant step closer to the quarter-finals.
Having seen Edinburgh win away to Racing Metro in Paris on Friday night, the Welsh side responded in the perfect manner at the Madejski Stadium.
Sam Warburton's second-half try complemented a fine Leigh Halfpenny kicking display as the Blues look to reach the knock-out stages for the first time since the 2008-09 season.
The result ended London Irish's slim hopes of progress from the pool and ruined head coach Toby Booth's ambitions of marking 10 years at the club with a victory.
Bath's disappointing European campaign continued as they slumped to their fourth defeat in five matches after going down 24-22 against Montpellier in Pool Three following a last-gasp try.
Bath had looked set for victory as two tries from lock Ryan Caldwell helped put them 22-17 ahead going into the final minute, but Montpellier snatched their first-ever Heineken Cup win when Timoci Nagusa went over for a score that was converted by Martin Bustos Moyano.
Matt Banahan also crossed for Bath while Alex Tulou and Pierre Berard scored the hosts' other two tries.
The result leaves Bath bottom of Pool Three on seven points.
Clermont ran in 12 tries to thrash Italian whipping boys Aironi 82-0 in their Pool Four match at Stadio Brianteo to keep alive hopes of a place in the quarter-finals.
The French side raced ahead with some early scores and were already 47-0 in front at the break.
Wingers Sitiveni Sivivatu and Julien Malzieu completed their hat-tricks in the second half, with Brock James kicking seven conversions to add to four from Morgan Parr.
Lee Byrne scored a brace, while Alexandre Lapandry, James and Elvis Vermeulen also touched down as the French side collected a bonus point.
Leicester are facing an early exit for only the fifth time in 15 Heineken Cup campaigns after they were battered into submission and suffered a club-record 41-7 European defeat to Ulster at Ravenhill on Friday night.
The Tigers now cannot win Pool Four - which offers an automatic quarter-final place - after Ulster moved six points above them.
Tigers will need a bonus point victory over Aironi at Welford Road to give themselves any chance in the race for two best runners-up spots, yet teams in other pools appear better placed.
Leicester were undone by two first-half tries from Ireland international wing Andrew Trimble, while Ulster's South African scrum-half Ruan Pienaar put the boot in with five penalties and three conversions for a 21-point haul before wing Craig Gilroy and replacement Paul Marshall added late tries.
The Ospreys kept alive their hopes of reaching the quarter-finals of with a six-try romp as they defeated Treviso 44-17 at the Liberty Stadium.
The hosts had secured the bonus point after just six minutes of the second half as a penalty try added to Ashley Beck's first-half brace and Tommy Bowe's interception score, with replacement Kahn Fotuali'i and a second penalty try completing the scoring as fly-half Dan Biggar kicked 12 points.
Phil Godman kicked a last-minute drop-goal for Edinburgh to clinch a 27-24 victory over Racing Metro and keep them at the top of Pool 2.
In a delicately poised group Edinburgh grabbed their second victory on the road to keep the pressure on Cardiff Blues as they hunt for their first quarter-final spot in eight years.
Back-row trio Ross Rennie, Netani Talei and David Denton all crossed as the visitors led for the majority of the game.
But Racing responded with three tries of their own and looked set to share the spoils before Godman struck to clinch only Edinburgh's second victory on French soil.
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