Trinidad and Tobago defender Brent Sancho joins on a two-year contract after finally obtaining a work permit for him.
The 28-year-old had spent time on trial at Kilmarnock and Brighton during the summer after leaving Dundee at the end of last season.
39-year-old Claridge recently trained with Sheffield Wednesday after being sacked as Millwall manager in July, but was not offered a deal by the Owls.
Gillingham are Claridge's 14th league club.