One of a group of Tory High Churchmen, he was on good terms with Francis Atterbury, George Smalridge and Robert Nelson, and one of the Commission of the end of the reign of Queen Anne.[6] In 1714, the year of her death, he became bishop of Chester through Harley's influence (consecrated on 14 April of that year in Somerset House Chapel). When Atterbury was put on trial in 1723, Gastrell spoke in his defence.[7]
He was a noted controversialist, but considered to hold moderate views. He criticised Anthony Collins, but was quite approving of John Locke.[8][9] He criticised the Trinitarian theories of William Sherlock as innovative.[10]
As a diocesan bishop, he is noted for the fine-grained survey he made of every parish, published in the nineteenth century as Notitia Cestriensis. Along with it was published the so-called Gastrell Manuscript, or Chronicle of St. Werburg, edited by F. R. Raines.[11]