Following Westley's sacking from Preston earlier in the year, Mousinho started the 2013–14 season having made just three appearances for the club in the opening three months of the campaign under new manager Simon Grayson. He was subsequently loaned out to fellow League One club Gillingham on 7 November 2013, on a two-month loan agreement. Mousinho made his Gillingham debut a day after signing, playing in a 1–1 home draw with Brackley Town in the FA Cup. He scored the winning goal for Gillingham against former club Stevenage on 26 November 2013, scoring in the 76th-minute in the match to ultimately give Gillingham a 3–2 victory. Mousinho suffered a hamstring injury in a 4–1 loss to Rotherham United four days later, and returned to his parent club prior to the end of the loan agreement. He made six appearances in his one month at Gillingham.