I tested this situation with your figures and TurboTax switched to itemized deductions. Let's see if we can figure out why it's not working for you.
How did you determine that "TurboTax still applies a standard deduction"? Did you look at the actual Form 1040? What is the amount on Form 1040 line 12?
Does the $300,000 of gambling winnings appear on Schedule 1 line 8b? If not, what form and line is the $300,000 entered on?
How did you enter the gambling losses? Did you enter it in the gambling winnings section of the interview?
You didn't say whether you are using TurboTax Online or the CD/Download TurboTax software. If you are using TurboTax Online, click this link for instructions to look at Form 1040 and Schedule 1. ("Preview my 1040" will also show you Schedule 1.) If you are using the CD/Download software there are a couple of other things you can check in forms mode.
Even if TurboTax is using the standard deduction, you can open Schedule A in forms mode. Do the gambling losses appear on Schedule A line 16?
Also in forms mode, open the Federal Information Worksheet. Scroll down to Part VI and look at the first three check boxes in that section, under "Standard Deduction/Itemized Deductions." Is any one of those three boxes checked? If so, which one?
