A product of the Crystal Palace youth system, Cronin went on to play for Wycombe, Oldham, and Shrewsbury before moving to Stonebridge Road in 2006.
He played 176 times for the Fleet - helping them win the FA Trophy in 2008.
Gillingham have the option of retaining Cronin for a further year if he impresses enough in his first season at Priestfield.
Source: BBC Sport
Source: BBC Sport