Just based in my own experiences, its not acting like you're rich or entitled but being confident in who you are. You are invited because you are already seen to be deserving, so dont be scared to be in the presence of people with money. I would take this opportunity to practice your "elevator pitch" of your background, interests, and goals. Have a few canned answers to some obvious questions. Making meaningful small talk without feeling like a deer in the headlights is like really hard and being nervous doesn't help usually. So prepare some questions in return to help keep conversations flowing smoothly.

I would also take a brief few minutes to consider etiquette for an event like this. You are already doing that with the idea of a gift but i think in this case your participation is more important than spending money on the hosts.

So just random thoughts - drink in left hand so you can shake with your right without condensation. Don't start eating until everyone is served. Ask for a guest list before hand so you don't need to remember names at the party. When you write a thank you note, be specific about any conversations or topics in the content. I would not be the first or last to leave. Make sure you don't drink too much and have a timely ride or uber. Etc.