oh my dd was invited to a party once, the mom, my friend, told me personally and clearly no gifts. I said OK, I believed her. EVERYONE else brought a gift

I felt sooo bad and cheap. Check with the others if you can to see what they are going to do. Otherwise, something thoughtful that doesn't cost much would be nice and meaningful....
ETA- I felt so bad that we actually went out and bought something and gave it to her the following week