The 28-year-old initially joined the club at the beginning of the month and did enough to travel to Canada with the squad for the Lions pre-season tour. However, after taking a closer look at him, boss Dennis Wise has decided not offer Farrelly a contract.
Speaking to the club's official website, assistant manager Ray Wilkins explained the reasoning behind the decision.
"We had Gareth on trial with us, he is a smashing lad and worked extremely hard whilst he was here. But unfortunately Gareth didn't quite offer what we are looking for. We would like to wish him all the best for the future."
The Lions, meanwhile, take on Southend United at Roots Hall this evening in a pre-season friendly and Wise will take a full squad to the Third Division club.
Squad: Stack, Lawrence, Cahill, Dolan, Ifill, Livermore, Harris, Dichio, Ward, Marshall, Weston, Wise, Morris, Elliott, Sweeney, Dunne, Cogan.