The war gratuity was introduced in December 1918 as a payment to be made to those men who had served in WW1 for a period of 6 months or more home service or for any length of service if a man had served overseas. The rules governing the gratuity were implemented under Army Order 17 of 1919.
Details of gratuities paid to deceased soldiers are shown in the soldiers effects registers. These registers are held by the National Army Museum but have recently been digitized by Ancestry (subscription service).
In most cases the war gratuity was paid to men in lieu of service gratuity due under the Royal Pay Warrant however, as the war gratuity was not introduced until 1918, many men had already been paid the service gratuity and therefore, when it was calculated, the war gratuity had to be adjusted so a man did not receive the full value of both.
The monies due were generally paid in to a Post Office Savings Account for each man who had to apply for savings book. A sample receipt can be seen here. If the man had already died or had been discharged insane an alternative payment method could be used.
A spreadsheet calculator for the war gratuity is available to download.