Stimson was sacked by the Kent club on Monday after their relegation to League Two on the final day of the season.
And chairman Paul Scally (pictured) has revealed that Barrett, who was Stimson's number two at Stevenage, has also departed along with former West Ham midfielder Robson.
Scally told Kent News: "They have all left the club, that's the end of it."
On Tuesday, Gillingham also released 14 players, including captain Barry Fuller.
Goalkeeper Simon Royce and defenders John Nutter, Garry Richards and Tom Wynter are all free to leave the club.
While midfielder Mark Bentley, Stuart Lewis, Adam Miller and Rashid Yussuff have also not been offered a new deal along with strikers Dennis Oli, James Walker, Luis Cumbers, Andy Pugh and Jacob Erskine.
But the club added that all 14 players will be assessed by the new manager once an appointment has been made, provided they have not been fixed up with other clubs.
Youth team striker Charlie Stimson has decided not to take up the offer of a professional contract, but Dean Rance and Connor Essam have put pen to paper on one-year deals.