Lions co-captain Andrews has been banned for three matches, while Gardiner will have to sit out the Swans meeting at the SCG on Saturday night.
It leaves the reigning premiers undermanned in defence as they attempt to blunt new Sydney spearhead Charlie Curnow.
Fellow Lions flag stars Hugh McCluggage (calf injury) and Zac Bailey, who did not contest his one-match ban for striking, are also unavailable this week.
Andrews gave evidence at the tribunal on Tuesday night, arguing he was attempting to shepherd for Dayne Zorko when he threw his left arm back and accidentally concussed Western Bulldogs forward Arthur Jones in the Lions' season-opening defeat.
He said he was unaware Zorko had gone to ground after the trio were involved in a marking contest and that the incident was a "very unfortunate accident" that occurred when Jones lunged at the ball behind him.
The AFL match review officer (MRO) graded the incident careless conduct, high contact and severe impact.
Brisbane, through legal representative Adrian Anderson, argued the action was not careless and did not warrant a three-match ban because the force was not severe.
But the tribunal found the attempted shepherd was clearly a careless strike and the force of the blow was severe.
"It left Jones motionless on the turf for a period of time and he suffered a concussion and immediate bruising below his eye," the tribunal said in its verdict.
"There is nothing about the impact that leads us to conclude that other factors relevant to impact are more consistent with a lesser impact grading.
"We find that this is a careless strike, high contact and severe impact."
Gardiner argued he was not guilty of striking because his high contact with Bulldogs forward Aaron Naughton did not constitute a strike.
Anderson said it was a double-handed push that resulted in glancing contact with the back of Gardiner's tricep to Naughton's neck.
The Lions also submitted the incident constituted a low-impact grading at most, noting Naughton's medical report showed a clean bill of health.
That incident was initially assessed by the MRO as high contact, medium impact and careless conduct.
The Lions compared it to an episode from last season, when Patrick Dangerfield's high contact with St Kilda's Ryan Byrnes was deemed low impact and the Geelong champion was fined for striking, rather than being suspended.
But the tribunal rejected Brisbane's arguments, finding the impact was more than negligible and Gardiner's action was a strike.
"We do not accept that it was in the end a push, although it may have started out as a pushing motion," the jury said in its verdict.
"Gardiner's right arm, in particular, extended up and we find that forceful contact was made by the forearm, elbow and tricep region with Naughton's neck and head.
"The impact was more than negligible. The frame-by-frame viewing shows forceful contact was made."
Bailey was banned over on a high elbow to Bulldogs debutant Michael Sellwood after the Lions star was tackled over the boundary line.
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