No. Your cost basis is $750,000, because you paid income tax* on that amount. If you sell for less, you will have a capital loss not a taxable gain. Depending on how you used the property, you may even have a deductible loss**.
* I assume by "gambling tax", you paid 25% federal income tax , through withholding or some other way. You report the $750,000 prize as income on your tax return and then claim the 25% withholding as a credit.
**If you used the property for personal use (home or 2nd home, including letting friends or relative live there), you may not deduct the loss. If it was treated as investment property (no personal use), you may deduct the loss.
