Arbitrary dismissal and basis for the calculation of gratuity

Good afternoon,

I have been working in a private firm for the last 13 years in an unlimited contract. Now, I have been terminated as per their part of downsizing. My notice period would complete by the end of April.

Since 2006, I'm working in the same company, starting my salary was 3500 and my basic was 2500 then upon increments they added all the increments in others category. They never changed the basic from 2500 and now my salary is 12000.

In 2017, they forced me to change my visa from the branch to head office license due to the branch license closure, and that time, they needed to change my basic as per my salary so they put 7500 as my basic.

During that of renewal, I signed a new employment contract mentioning the new basic, I have the document with me as well.

My question is, am I eligible for 3 months' salary and whether these 3 months include the notice period or apart from that is 3 months?

Will my gratuity be calculated based on the new basic or old basic salary?

Thank you.