Is it ok to sign my cancellation even my graduity...

The gratuity shall be paid within ten days after work termination, and canceling the visa is not a requirement to pay the gratuity
We would like to inform you that according to the UAE Labor Law, the old contract is as extension to the new contract, but note that the end of services will be calculated according to the last contract.
Article 132
A worker who has completed one or more years of continuous service shall be entitled to severance pay at the end of his employment. The days of absence from work without pay shall not be included in calculating the period of service. The severance pay shall be calculated as follows: 1. 21 days' wage for each of the first five years of service. 2. 30 days’ wage for each additional year of service provided always that the aggregate amount of severance pay should not exceed two year's wage.
Article 133
A worker shall be entitled to severance pay for any fraction of a year he actually served, provided that he has completed one year of continuous service.
Article 134
Without prejudice to the provisions of laws that grant pensions or retirement benefits to employees in certain firms, severance pay shall be calculated on the basis of the wage last due for monthly, weekly and daily paid workers, and on the basis of the average daily wage referred to in Article 57 hereof for those paid on piecemeal. The wage used as a basis for calculating severance pay shall not include whatever is given to the worker in kind, housing allowance, transport allowance, travel allowance, overtime pay, representation allowance, cashier’s allowances, children education allowance, allowances for recreational and social facilities, and any other bonuses or allowances.
Therefore, according to the ;aw you can claim for the end of services as the last contract stated.