William the Conqueror granted the Manor of Deptford or West Greenwich to Gilbert Magminot or Maminot, bishop of Lisieux, one of the eight barons associated with John de Fiennes for the defence of Dover Castle. These eight barons had to provide between them 112 soldiers, 25 of whom were always to be on duty within the castle, and the rest to be ready for any emergency. Gilbert de Maminot’s share of the lands amounted to 24 knight’s fees as follows: two in Pevington, two in Eastwell, two in Davington, one in Cocklescombe, three in Thornden, three in Waldershare, two in Kennington, two in Cowdham, one in Billingham, one in Hartwell, one of Hugh Cheriton, one of Ralph Rovery, two of Roger Barham and one of Simon Wellard, which together made up the barony of Maminot, held at Deptford as the head of the barony.
In 1814 John Lyon wrote that Maminot built a castle, or castellated mansion, for himself at Deptford, of which all traces had by then long since been buried in their ruins, but from the remains of some ancient foundations which had been discovered the site was probably on the brow of Broomfield, near the Mast Dock and adjacent to Sayes Court.
There is little known about de Magminot after he was appointed Marshal of Dover Castle. He was said to have had a son, Hugh, from whom Walkelin Maminot, who held Dover Castle against the Empress Maud was descended.