I thought gratuity was based on total pay but apparently it's capped at 2 years' worth

I'm in my eighth year here, and since my child moves up a grade next year I have to decide where we'll be within this year, so I'm starting with the end-of-service gratuity. Some people say it's calculated on basic salary only rather than total pay, and I vaguely heard there's a cap, but the company still hasn't told me the exact basis. I was told that delaying or moving up our return date because of my child's school year could change the settlement amount, so I want to confirm this first.