Dear Bikash,
Clarification for 1: Gratuity years mean the number of years of service done by the employee in the company. For example, Mr. A joined the company on 1.12.2000 and left the services on 17/08/2010. His total service years are 9 years and 9 months. This is called the number of years of service for the employee. It is rounded to 10 years as the month is over and above 6; it should be counted as one year - this is called gratuity years. His last drawn salary or the salary at the time of resignation is Rs. 26,000 per month.
Clarification 2: Calculation of Gratuity: 26,000 x 15 x 10 divided by 26. Then the amount of gratuity will be Rs. 150,000.
Clarification 3: Since continuous service is emphasized in the Gratuity Act, it is applicable only for regular employees. As you mentioned, employees paid weekly wages, their salary for Sunday is not calculated, and they are treated as casual employees; for those, gratuity is not applicable. But if they are treated as permanent employees on your rolls by considering the continuous service, they are also eligible for gratuity.
Clarification 4: Yearly 15 days mean that 15 days' salary for a year of service is paid as gratuity. I hope this clarifies it for you.
P. Ramachandran