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

You could say 'please contribute cash gifts only, if possible. 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 either say " I'd appreciate anything you give me as a gift and I prefer gift cards or cash" but be sure to tell them why.  Is it because their money on a gift for you would be spent better that way, then for instance spending it on a sweater that didn't fit you and you had to return it?  When making returns if you don't have a receipt you will probably get store credit or just be out of luck.  If this is someone you exchange gifts with every year then be sure to ask them also--how they would like to receive a gift from you.

If this is someone you only exchange gifts with---rarely---then I wouldn't say anything at all.  Just accept your gift, graciously.

Just simply say please cash or gift cards.