As a
Presbyterian, Gell's sympathies generally lay with Parliament, but the importance of the Derbyshire lead mines for manufacturing ammunition meant Charles unsuccessfully tried to win his support. When the war began in August 1642, Gell was instructed to secure Derbyshire for Parliament and recruited a regiment with his brother
Thomas as
lieutenant colonel. The local Royalists were led by Chesterfield; when he ransacked Hopton Hall, Gell retaliated by sacking his residence at
Bretby Hall. He followed up by plundering
Elvaston Castle, former residence of Sir John Stanhope, allegedly defacing his tomb in the local church and digging up his flower beds. In March 1643, Gell joined
Lord Brooke in an attack on
Lichfield Cathedral, which was occupied by a Royalist garrison under Chesterfield. After Brooke was killed by a sniper, Gell took command and captured the position along with Chesterfield, who was held in
London until 1649. He then combined with
Sir William Brereton in an attack on
Stafford, which ended in the inconclusive
Battle of Hopton Heath on 19 March. The Parliamentarian artillery was captured but the Royalist commander, the
Earl of Northampton, was killed after refusing to surrender. Gell offered to hand over the body in return for his artillery; when this was refused, he had Northampton's embalmed corpse paraded through the streets of Derby before burial. While Lichfield was soon retaken by the Royalists, Gell replaced Brooke as Parliamentarian commander in
Staffordshire and
Warwickshire and was appointed Governor of
Derby in 1644. Although an energetic and capable soldier, his bullying of the Derbyshire county committee and the plundering for which his unpaid troops became notorious provoked numerous complaints to Parliament. The formation of the
New Model Army in April 1645 reduced his influence while he resented Derbyshire being used as a base by its troops. He reportedly ignored orders from Sir
Thomas Fairfax to bring his troops to
Naseby in June, then was reprimanded for allowing 3,000 cavalry from the defeated Royalist army to escape him at
Ashbourne. As the war came to an end in April 1646, it was discovered that he was negotiating more favourable terms with the Royalist garrison of
Tutbury Castle than those offered by his colleague Sir William Brereton. He resigned his commission and went to London seeking payment of expenses he claimed were owed him; as a precaution, he first signed over his estates to his son John, keeping an annuity of £1,100 for himself. ==After 1646==