The law says you have to pay taxes on all gambling winnings. You are allowed to offset your winnings with your losses, but you are also required to keep complete records. If you always use your player tracking card, the casinos will provide you with this documentation. If you have more losses than winnings, you cannot claim a deduction.

The W-2G is a requirement for the CASINO to report large wins, it doesn't mean those are the only winnings you owe taxes on. But then you knew that, right? You haven't been cheating on your taxes, have you?