Gratuity after inter branch transfer

hi

I am working with a school since Jan 2018 in gurgaon (it's 4years and 8months)

now school management want to change my appointment from school to pvt ltd company ( internal group company ).

management of school and pvt ltd company are same.

he assured me to pay gratuity either my appointment in school or in pvt ltd company and he also will confirm this over email.

now my question is that is it possible that I will eligible for gratuity or not in this situation.

please help to confirm if any document or confirmation I need to get from school or pvt ltd for safer side.