Why is FRCS written after Mr. Gault’s name?

The letters FRCS after a surgeons name indicate that he/she is a fellow of the Royal College of Surgeons of England.

“The letters FRCS after a surgeon's name mean that the surgeon's education and training, professional qualifications, and surgical competence have passed a rigorous evaluation, and have been found to be consistent with the high standards established and demanded by the College, of which the surgeon is a current Fellow”.

“Some surgeons only use the highest of their qualifications (eg. FRCS) in their correspondence or on their nameplates, rather than listing all the lesser degrees as well”. Source: Royal College of Surgeons

Why is Mr. Gault not referred to as Doctor?

Surgeons with the FRCS qualification are known as Misters (Mr) as a result of British traditions in medical practice stretching as far back as the Middle Ages. Surgeons in the UK gain the title 'Dr' on completion of their medical school degree. Thereafter a further period of postgraduate study and training through junior posts is required before sitting exams to become a Fellow of the Royal College of Surgeons. On gaining this degree the 'Dr' earns the right to drop the title and become 'Mr' again. See here for a detailed explanation on the Royal College of Surgeons website. David Gault FRCS dropped the title Dr. to become Mr. Gault again in 1982.