Hi,
The definition of employees under Payment of Gratuity Act is as under
"employee" means any person (other than an apprentice) employed on wages, in any establishment, factory, mine, oilfield, plantation, port, railway company or shop, to do any skilled, semi-skilled, or unskilled, manual, supervisory, technical or clerical work, whether the terms of such employment are express or implied, and whether or not such person is employed in a managerial or administrative capacity, but does not include any such person who holds a post under the Central Government or a State Government and is governed by any other Act or by any rules providing for payment of gratuity.
The Gratuity would be calculated from the Date of Joining, not from the date of confirmation of probation.
In your case, you are keeping an employee as trainee for 6 years, then put him of probation.
You have to calculate the Gratuity from the date of joining.
AMATYA