Defenders Josh Gowling, John Nutter and Tony Sinclair have also been told their contracts will not be renewed.
Strikers Andy White and Mark McCammon make up the eight to leave.
Julian, 28, played 39 of the 46 league games this season having joined three seasons ago from Stevenage. Cronin made just seven appearances following his summer move from Ebbsfleet United.
Bentley's departure ends a five-year association with the club, in which time he has seen one promotion and two relegations and spent a spell as club captain.
The 33-year-old midfielder played just under 200 games for the Gills.
Left-back Nutter played 37 games this season but lost his place in March to Joe Martin, who was playing as a non-contract player but is thought has been offered a deal.
Gowling and Sinclair have found it hard to command a regular place in the Gills defence this season. Gowling has been with Gillingham for two years while Sinclair was a summer signing from Woking.
White made just two substitute appearances for the Gills, against Norwich City in the Carling Cup in August and Barnet in April. McCammon ends a disappointing spell with the club in which he scored just five goals in three years, the last of them coming at Chesterfield in April 2009.
There has been no announcement about striker Bayo Akinfenwa and wide players Andy Barcham, Joe Martin and Chris Palmer, who are also out of contract, but it is believed they may have been offered new deals.