Do i need to pay taxes on a gift in excess of $200,000 given to me by my aunt?

The receiver of a gift never pays taxes on the gifted amount no matter how much it is.  Now it sounds like you are trying to buy a home and the underwriter wants the gift to be in writing ... ask them if you have any further questions.

Now your aunt will need to file a gift tax return since she exceeded the $14K per year limit to any one person. 

"I was also told that I must have a letter from the donor.... I will have to pay the taxes."
No, that's not true.
As Critter#2 indicates, if the mortgage company is asking questions, you need to address their concerns (usually that it's really a gift and not a loan). But, it's not taxable to you.