Payment of gratuity

Joined in 1994 as an assistant in non muster rolls and continued up to 2001 feb, from 2001 , feb continued as assistant on 89 days employment in Jul,2008 management confirmed services as an assistant and promoted as Supervisor in 2012 and worked up to 2014 Nov, and resigned from services, but management has paid gratuity for 6 years of confirmed services, can gratuity be claimed from 1994 onwards since documentary evidence is available basing on the Payment Vouchers and other tour reports from 2001 feb salary slips are also available total service is 20 years 4 months, but company has provided me Service Certificate for only 6 years due to which I have to forego a valuable career of 14 years.. Can we sue the company under defamation case and denying natural right of getting Service Certificate for 20 years ??