I'm not from an observant family or branch of Judaism, so I have actually only attended one bris and it was a LONG time ago. As I recall, there were no gifts at that event, but my memory is hazy. Then again, now that I'm older and have kids, I'm thinking that since this ritual is similar to an infant's baptism (bringing a child into a faith), it would seem perfectly appropriate to give the parents something--a gift card to their favorite bookstore or baby- stuff store, a donation to the synagogue in honor of the event, ... depends what you think would be meaningful to them.
Other ways to get more information...Do you know any other people who have been invited? Maybe you could ask them what they're doing? Another possibility is to call a local synagogue (the yellow pages should have a section for them) and ask whether they can point you towards something like an online etiquette guide for Jewish life events--in the course of explaining your question, they may just answer it for you. Make it clear you are not presuming that it's their job to educate you about Judaism and Jewish culture, just that you hope they'll help you educate yourself. It's lovely that these folks are including you in what is indeed a very important moment. I hope you enjoy the company and celebrating (which as you know will last much longer than the actual cutting).