Under normal circumstances, when you get such a letter, my response would be to prepare an amended return reflecting whatever you should have reported (in this case, gambling winnings and losses), and mail the amended return plus a detailed letter of explanation to the office that sent the notice, rather than the address for amended returns. Given your interactions so far, I would still suggest that course of action. (Even if your losses exceed your winnings, you may owe tax because losses are a schedule A itemized deduction which is sometimes limited.)
