The 24-year-old has returned to the County Ground for a third spell having first joined the club in 2013 before leaving for QPR in 2015.
Gladwin re-joined the Robins for a loan spell last season and will bolster their ranks for the second half of the current campaign, with the midfielder revealing that the decision to join Luke Williams' side was an easy one.
"The best place to come and play would always be here and it was a very easy decision," Gladwin told the club's official website.
"I've got great memories here. I'm really looking forward to Bolton on Saturday. They are a huge club and hopefully we can go there and get the job done
Gladwin will be available for Saturday's clash against Bolton.