By his own words the poster has been in the continuous service of the establishment for 28 years from 1992 to 2020 and retired in 2020. Gratuity being a terminal benefit, it would become payable only in 2020 on his retirement. When gratuity thus payable on termination of employment, no deductions could be made from it. Then the 1 lakh rupees he received in 2014 can only be an advance or loan and cannot be a part of his gratuity even it was agreed to be so by his employer and himself.
The post is incomplete with the absence of vital details like whether the PG Act,1972 is applicable to the establishment any time during the poster's tenure, his last drawn salary.
However, if the Act is applicable, he is entitled to gratuity for the entire stretch of his 28 years of service in the organization based on his last drawn salary without any deduction. The employer cannot refuse to pay on the reason of the amount already received nor they can adjust it now against the statutory gratuity. It is for the poster to pay back the amount after receipt of his gratuity. Since there is denial to pay gratuity by the employer, the poster can file a claim before the Controlling Authority under the Act for recovery.