The 26-year-old Silhillian becomes Monk's fifth summer signing.BCFC
With David Davis out for an extended period of time with an ankle injury, Garry Monk had been looking to increase his options in the middle of the park.
The 26-year-old Silhillian becomes Monk's fifth summer signing.
Former England Under-21 international Gardner joins his older brother Craig in the playing ranks at Blues.
Gary is a product of Villa's Academy and made his senior bow for the club in December 2011.
Since that time, he has spent spells out on loan with Coventry City, Sheffield Wednesday, Brighton & Hove Albion and Nottingham Forest.
And last season saw him spend most of the campaign with Barnsley.
Gardner will wear the number 20 shirt during his time with Blues.