If the 'gratuity' is a payment from your previous Indian company made when you left them, then the US tax system will treat it exactly the same as wages paid by your previous company.
Whether or not you need to pay taxes on your wages and gratuity will depend on whether your are considered resident in the US for tax purposes for this financial year. It is likely that you will be. Assuming you are, then the US requires that you pay tax on all income, wherever it is earned in the world. You will need to fill in a tax return and declare both your gratuity and your wages in India for that year.
India and the US have a 'double tax agreement', which means essentially that you won't be taxed twice if you have already paid tax on the gratuity and wages in India. But you do have to declare them.