A gratuity salary is a sum of money that an employer gives to an employee after around 5 years of work with the company.
If you've worked for the company for at least 5 years and have a good work history with the employer then after the 5 years the employer will sometimes give an employee a gratuity payment.
That gratuity is basically just a payment like a tip you would tip a waiter.
The gratuity can also be seen as like a bonus on top of your normal salary but a gratuity from my experience is just a once a year or so payment.
When i worked for a company they gave me a lump sum of $3,000.00 for the year as a gratuity since I was a good worker for them.
At some other companies I never got a gratuity.