Right now, this field is not available. It would be ideal to allow this, since we use a date in our letters. Using the MS Word date autofield isn't a valid option - whenever you open the letter, that date updates, and it doesn't give you a good idea of when the letter was printed. Gift date may not match the date you're running the letters.
??? How could you add it as a field when the letters have not processed to even give you the data if it were an option? When you say you "use a date in your letters" do you mean related to the gift or the standard date heading in a letter? I don't use the autodate field for my letters because as you said it updates when you open the file. I just do the actual date, not autofilled, and then search and replace for future merges.
I agree - the data in that field would not yet be populated. However, since the gift acknowledgement date is populated when you run the acknowledgement letters, it could be provided as-if it were, simply by providing the current date when generating the letters.
You can set the "Autodate" to not update automatically in the merged document: http://www.pcmag.com/article2/0,2817,2349189,00.asp