Do I have to amend my tax return if I used the standard deduction and did not declare gambling winnings?

I did not have enough deductions to itemize so I used the standard deduction but did not see where to put in info about gambling winnings. I sent for my refund and have received it. Do I have to amend my taxes to account for around $400 dollars I think I owe for gambling winnings even though L lost  more than I won.