Gratuity and multicompany firm process

Respected sir, My Name is Satish Madhukar Dewajwar , I have join a private limited company (construction base) on 07.07.2016 after that they transfer to me another their firm on 01.04.2011 i have transfer later so i am working in another company as my company transferred but my job is continue under one group of multicompanies but now in in December 2017 my company transferring again me in first company but they want my resignation and also they dont want to calculate first company as continue period for graduity so please help me what to do. My service period is 11 years with both companies but company calculating Gratuity from 01.04.2011 so please help me what to do and why need this time appointment letter and resignation later i dont no to transfer previous company.