It is not always easy to date Anselm's letters, but it seems likely that Gilbert must have gone to Canterbury at the close of the year 1079, shortly after Anselm's return from his first visit to England. Of the time which he spent in Lanfranc's service we know nothing; but that he had proved his worth we may conclude from the fact that it was at Lanfranc's instigation that the king appointed him to Westminster.

The date of his appointment to Westminster cannot be fixed with certainty. There is good evidence for placing his death in 1117; and, if we may trust the writer of the Miraculum, he was abbot for thirty-two years. This would give us 1085 as the year of his accession: and with this as a provisional date we must at present be content.

The Conqueror's reign was nearing its close. The great Domesday Survey was being made when the new abbot was fresh in his seat; and he doubtless attended the meeting at Salisbury on 1 August, 1086, and took the oath to be King William's man against all his enemies. Later in the same year the king left England for the last time. On 26 September, 1087, Gilbert must have helped his old master, the aged Lanfranc, to crown William Rufus, and have heard the welcome pledge which the new king gave that in all matters he would be guided by the archbishop's advice.