The Jamaica international made over 400 appearances for Wanderers in a 14-year stay at the Reebok Stadium but played only six times in each of his last two seasons with the club before being released in the summer.
Persistent injury woes were a large factor in Gardner's struggle for games and Coyle has given the 34-year-old the opportunity to train with his former team-mates in a bid to put those problems behind him.
"I am just trying to stay fit and be patient," said Gardner. "I'll wait for the right opportunity to arrive.
"I am grateful to be training here with the gaffer and the team. He didn't need to let me, but it shows how much of a good person he is.
"I am training on the pitch in the morning and then going to the gym in the afternoon to try and get my legs right. Football is what I love doing but it has been frustrating being out injured for that long and not being able to play."