Players who signed at age 18 or younger are exempt for five years while playes who signed at age 19 or older are exempt for only four years. The date 40 man rosters had to be set used to be November 20th, but I'm not sure if that changed in the last CBA (the years exempt did change).
Gardner signed in August, 2003, at the age of 21, meaning 2007 is his fourth (and presumably last) exemption year. It looks to me that Gardner will have to be added to the 40 man roster by this November if the Reds want to protect him from the Rule 5 draft this offseason.