A garrison sergeant major (GSM) in the British Army is the senior warrant officer of a garrison and holds the rank of warrant officer class 1. The GSM London District, always a guardsman, holds one of the four most senior WO1 appointments in the British Army, and has military ceremonial responsibility for important state occasions such as Trooping the Colour.
Garrison sergeant major Wikipedia
The post of GSM London District was established in the early 1950s with specific responsibilities as State Ceremonial Sergeant Major. The first task of the new GSM was to organise the funeral of King George VI in 1952 and the coronation of Her Majesty The Queen in 1953.
Traditionally the GSM London District wore the same badge of rank as a regimental sergeant major of Foot Guards, the large Royal Coat of Arms on the right upper sleeve. However, on 28 April 2011, the day before the wedding of The Duke & Duchess of Cambridge, the Ministry of Defence announced that, in recognition of the work done by garrison sergeant majors on behalf of the Royal Household, Her Majesty the Queen had approved the revival of the original insignia made for sergeant majors appointed to the Court of King William IV in the early 19th century. It incorporates the large Royal Coat of Arms worn by selected warrant officers class 1 of The Household Division, placed over four chevrons sewn in gold thread, the traditional badge of the sergeant major, originally worn on both arms of their tunics.