Attorney-General of the Isle of Man, and the lawyer whose reading of the Manx constitution later writers defer to.

He began as High Bailiff of Castletown, and in that character edited Deemster Parr's Abstract of the Laws, Customs and Ordinances of the Isle of Mann for the Manx Society from the unpublished manuscript. He thought Parr's work the standard authority as to the common law of the Island, and regretted that it had never been printed.

His own account of the legal side of the constitution became the standard in its turn. A modern writer, declining to argue the Atholl compensation disputes over again, says simply that the legal aspects of the constitution have been made the subject of a single work by Sir James Gell and that he has commented on the disputes adequately already. Moore and Walpole both quote him as an authority on points of Manx history rather than merely of law. He judged that the Stanley succession case was purposely protracted until 1609, when the youngest of Earl Ferdinando's daughters came of age. He dismissed Seacome's story that Earl William had been so long abroad that few could identify him on his return, on the ground that William Stanley had been governor of the Island from 1592 to 1594 and was therefore perfectly well known there.

As Attorney-General he fought the Island's corner on the reach of its own legislature. The Law Officers in England had advised that the Insular Legislature had no power to legislate on a particular matter even with the consent of the Crown, and that an Act of the Imperial Parliament was needed. Gell prepared the case in answer, resting on the numerous precedents of insular legislation affecting Crown rights.

He sat as a judge as well. In one case under an Act of 1691 he held that where a brewery had lent money to a publican and in the same deed tied him to buy its beer, the profit on the beer was an indirect return upon the loan, and the whole loan, principal and interest, was void.

He was a public figure in the ordinary way too. In February 1872, as Attorney-General, he attended the thanksgiving service at St Paul's for the recovery of the Prince of Wales with the senior member of the Keys and the Speaker, the three of them driving there in a carriage with the Three Legs painted on the panel, a symbol then almost unknown in London which attracted considerable attention. He was elected one of the lay representatives of the diocese of Sodor and Man on the Central Council of Diocesan Conferences. His firm, Gell and Gill, formed with his second cousin who had studied law with him and was admitted to the Manx bar in 1864, did a very large business until the partnership ended about 1882.