As you are eligible for grauity see the work out as you completed 8 years of service means they see how many days you worked in a year
240 days comes into picture for deciding whether an employee was on continuous service or not and as such a person who had worked for 240 days (above the ground or 190 days below the ground) in a year is deemed to have continuous service for that year.
The provisions relating to six days week and 190 days of work was provided after the ruling by the apex court in Lalappa Lingappa Vs Laxmi Vishnu Texiles Mills, Sholapur, to safeguard the interests of those employees who would be denied gratuity for want of minimum days (240) in between the years. Here also completion of 5 years is mandatory. Once eligible for gratuity, if in any year one has not worked for 240 days, that year can be counted as continuous provided he was working for at least six days in a week and had worked at least for 190 days in the preceeding year.