Centre-back Gardner, who has been capped once by England, spent the latter part of last season on loan at Everton but failed to make an appearance.
His deal completes a busy day for Brown, who also brought in Marlon King from Wigan on a season-long loan.
Hull have one more loan available, but Brown would not be drawn on whether he had created the space to try to bring Manchester United striker Frazier Campbell back to the KC Stadium.
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