Apparently there's some family history here of which we are not aware.
When I write a check and it's a gift, I write "Gift" in the memo line of the check. It's that simple.
The only time I've ever written a letter to explain it was a gift was when I gave some money to the then-Mayor of Durham, NC to help defray his personal expenses due to a recall petition. I did so only because checks written to politions are presumed to be campaign contributions.
Why was there a recall petition? He said "People should be able to live without fear of violence." Yes, that's why some nitwits started a recall petition against him. That and because he said it in a speech at a Gay Pride Rally. Thankfully there were not enough nitwits in Durham to garner the needed signatures.
Dick