A Cathedral exists because it contains a Cathedra, the Bishop's chair. The cathedral is therefore the mother church of the Diocese.
There are 43 Diocese in England, and each has a Bishop and a Cathedral.
Gloucester Cathedral hosts many diocesan events, such as confirmations, quiet days, and the annual ordinations of deacons and priests.
The Diocesan Offices are in Church House, College Green, which is attached to the Cathedral.