The 27-year-old Blues forward, who has struggled to command a regular starting place in the first team this season, has moved to Elland Road on loan for the remainder of the season.
"Gary brings plenty of experience and he is a good signing," Leeds manager Simon Grayson told the official Leeeds United website.
"He's coming from the Premier League to League One and he will add quality to the squad.
"He is a threat, he can play in a number of positions, and we're delighted to have him here."