How Do I Say, "No Gifts, Only Cash Or Gift Cards"?

Well, u could say that, only add please and thank you.
"No gifts, PLEASE. Just money or gift cards, THANK YOU."

You don't.  It's rude.  No one is obligated to give you a gift, let alone give you what you tell them to give you.  Even if someone asked me what I wanted, I would never in a million years say cash.  Your event is not a fundraising occasion that others need to contribute to.

You could say 'please contribute cash gifts only, if possible. Thank you'