Can I use funds gifted from my boyfriend to meet the financial requirements for a Destination Thailand Visa (DTV)?

If you are applying through NYC USA consulate they required a 6 month course FYI.

Some consulates accept shorter courses

You don't need to provide anything other than a bank statement in your name with 500k thb. Keep it simple

I don’t think a letter is needed. The Embassy does not need to know where the money came from. Not my personal experience, I read on some posts here (if you search with a keyword “500,000” or funds) people transfer the required amount from a brokerage account to their main account days before a monthly statement is issued so when they download the monthly statement the ending balance would show 500K baht. Personally, I submitted 3 monthly statements showing the required amount. Each embassy has their own interpretation of the requirement but to be safe, I would submit 3 monthly statements showing the required balance. Btw in my case I did not show the details, just a screenshot of the summary page. Good luck!