West Brom's Darren Moore scored twice heading into his own goal before grabbing an equaliser leaving Gaardsoe to stab the ball home.
Paddy Kenny finger tipped a Jason Koumas corner away with Lee Hughes closing as West Brom probed for an early opening. The breakthrough almost came in the 13th minute, Koumas given space on the right, picked out Geoff Horsfield who powered his header over from eight yards.
Koumas caused problems 10 minutes after Lee Hughes flicked the ball into his path, with a surging run towards the United area with three defenders in pursuit, but with Kenny charging off his line he tried to steer the ball round him and prodded it wide of the far post.
United's first real chance came on 30 minutes, in form Paul Peschisolido weaving his way towards goal from the left flank, his right foot shot pushed round the post and into the side netting by Joe Murphy.
Full back Rob Kozluk was inches away from grabbing a spectacular opener minutes later. Michael Tonge's corner was headed clear to the defender lurking out side the area and from 25 yards his right foot valley zipped wide of the target.
Hughes was handed a better chance minutes before the interval racing clear of his marker but his fierce shot was powered too close to Kenny who dived to his right to smother the ball.
United swept ahead in the 57th minute, midfielder Nick Montgomery lashed in a low cross and the ball crashed against defender Darren Moore's chest and over the line as Peschisolido made sure.
Moore made amends in the 70th minute climbing to meet a Koumas corner to finish with a firm header, the ball taking a slight deflection to spin beyond Kenny's reach Kenny made a spectacular stop from substitute Sean Gregan but was helpless as Gaardsoe swept the ball over the line five minutes from time after substitute Delroy Facey had headed back into the goalmouth.
Sheff Utd: Kenny, Phil Jagielka, Page, Whitlow, Kozluk, Ndlovu, McCall, Montgomery, Tonge, Ward, Peschisolido.
Subs: Shaw, Parkinson, Allison, Robinson, Boussatta.
West Brom: Murphy, Haas, Moore, Gaardsoe, N'Dour, Koumas, Kinsella, Johnson, Clement, Horsfield, Hughes.
Subs: Gregan, Volmer, Dyer, Facey, Pressman.
Referee: M Jones (Cheshire).