The 28-year-old, who spent two years with the Robins between 2006 and 2008, fills the void left by Jamie Cureton's injury which left Mark Yates' men short of attacking options ahead of Saturday's League Two clash with Plymouth.
"Steve knows where the goal is and that's what we need," Yates told Cheltenham's official website.
"I've been talking with him all week and I'm tried to bring him here on a few occasions. He's not match fit but we'll see how he goes ahead of the weekend."
Gillespie added: "I'm here to enjoy my football.
"It has been frustrating at Fleetwood but I'm looking forward to being back at Cheltenham and hopefully getting some games under my belt. I just want to play and when I do I will be ready."
Gillespie began his career with Liverpool and made 69 appearances for the Robins in his first spell having joined from Bristol City. He went on to spent four years with Colchester before linking up with the Cod Army last summer, but his first-team opportunities have been limited.