He got on the end of a Sekou Baradji free-kick to stab the ball past Sheffield United goalkeeper Paddy Kenny, to become the Royals' 11th goalscorer of the campaign.
Kenny was lucky to be on the pitch after he was only shown a yellow card when he handled the ball outside his area in the first half. The decision was particularly bitter for Reading as the Blades had no substitute keeper on the bench.
Reading opened the scoring after just three minutes with a goal that came straight off the training ground.
They won two corners in quick succession and from the second, taken by Glen Little, Gunnarsson sneaked into the penalty area to shoot beyond Kenny.
The equaliser followed just 12 minutes later when Steven Kabba headed in his seventh goal of the season, after getting on the end of an Alan Quinn cross.
Controversial United manager Neil Warnock was in trouble with referee Grant Hegley early in the game. Warnock stormed to the edge of the technical area to dispute a decision and after shouting and gesticulating at the referee was given a severe ticking off.
Warnock looked to be in trouble again after making a comment to Reading player Bobby Convey from the sidelines. That prompted home defender Stephen Hunt to confront Warnock, but this time Mr Hegley took no action.
The big controversy of the game came in the 33rd minute, when Kenny handled the ball outside his area. He was surprised by a back header from team-mate Nick Montgomery and stuck his right arm out instinctively.
Initially Mr Hegley took no action but after his assistant's signal he awarded a free-kick on the edge of the area and showed Kenny a yellow card when most spectators expected to see red.
Man of the Match - Steve Kabba
Kabba posed a constant threat to the home defence and his goal was a deserved reward for a busy performance.