The 32-year-old spent three seasons with the Grecians between 2006 and 2009, helping the club back into the Football League and playing a crucial role in their back-to-back promotions.
And Gill, who made 148 appearances for Exeter during his first spell, will be reunited with former manager Paul Tisdale until January.
Tisdale told the club's official website: "Matt is someone who knows his way around the club already and he will understand the core elements of the way we play. He should slot into the team seamlessly."
Gill is available for the trip to Scunthorpe on Saturday and will provide cover for injured midfielder Matt Oakley.