Gradwell was born at Clifton-in-the Fylde, Lancashire. He went to the English College, Douai in 1791. The college being suppressed by the French revolutionists, he was confined for some time, and was not allowed to return to England till 1795. With most of the Douai refugees, he went to Crook Hall, Durham, where he was ordained priest in 1802. He taught poetry and rhetoric for seven years at Crook Hall, and at Ushaw College. About this time, Pope Pius VII decided to reopen the English College, Rome, and on John Lingard's recommendation, Gradwell was appointed rector (1818).
Under his administration the establishment flourished. He also acted as Roman agent for the English vicars Apostolic, exhibiting tact and diplomacy in this office. In 1821 the pope made him a doctor of divinity. In 1828 he was consecrated Bishop of Lydda, as coadjutor to James Bramston, the vicar Apostolic of the London district, and he came to London soon afterwards to take up his new duties. After some years of ill-health, he died of dropsy in London.
His writings include: "A dissertation of the Fable of Papal Antichrists" (London, 1816); "A Winter Evening Dialogue ... or, Thoughts on the Rule of Faith" (London, 1816); and various journals, letters, and MSS. in connexion with his residence in Rome; his notes on the old archives of the English College there are of historical interest; all are in the Westminster archdiocesan archives.