Ha ha! I think you are a great black Emily Post :-)
Back when I was working, any time someone had a birthday, we would get a group together to take this person out to lunch. Everyone had to pay their own meal and chip in for the birthday person's meal. Unless is was a cheap gift or a gag gift, we didn't do gifts.
I'm guessing that your co-worker's party is occurring on a non-work day. And it sounds like an actual party where gifts are expected. I agree that this is tacky. I had a friend who used to throw a birthday party for herself every year and expect everyone to foot the bill. Every year, she had fewer and fewer people show up. To this day, she probably has no idea why no one wants to help her celebrate her birthday.
If you really really like this co-worker and genuinely want to help her celebrate her big day, then go ahead and go to the brunch. Otherwise, just decline the invitation and wish her a happy birthday. I personally decline these types of invitations because I get annoyed by bad etiquette.