I'd rather receive an invitation to something and think "huh? why did s/he invite me to this?" than discover that I was the only person out of an entire group not invited (barring toxicity between 2 people).
I can't say I've ever received an invitation and immediately jumped to the gift grab train of thought. The present (if required) is usually low down on the ladder after things such as availability, overall desire to attend etc.
In this particular situation I'd either think "that's nice of coworker to invite me to her event. Maybe she wants us to become better acquainted" or, if I thought it was strange, I'd just think "weird", and then RSVP no gracefully.
As it stands now, graduate doesn't come out looking too good at excluding one person, even if she is being used as a pawn by the other coworkers.
I agree with this. Especially in the case where literally everyone is invited. Some would be close enough to want to give an expensive gift, perhaps, while some would not feel close, but the feelings would range widely. If someone
I did not know very well but had no negative feelings toward invited me to something like this, I would probably use it as a chance to try to get to know them better by giving a card and maybe a token gift (I'm talking something small enough to fit in a card, probably, like along the lines of a bookmark).
But that would be under normal circumstances, and the circumstances of the OP are, to put it charitably, weird.
And, believe it or not, given the other posts I have been making here, I really, really do agree with this last sentence. If only some co-workers were invited, that would be one thing, and easily explained by her knowing some better than others, an attempt to keep the guest list manageable, or any number of other reasons as suggested by other posters.
But this? This looks awful. She really doesn't come out looking good at all.
And for exactly that reason, I would suggest that the OP try to see what's going on. If GCW is just naive and has somehow been manipulated into this, it's not very kind to her to tar her with the same brush as the other co-workers. It isn't required, but I think it would be kind to somehow address her privately with at least a card, and help her to understand how badly what she did could be taken. And that there aren't any hard feelings.
On the other hand, it's entirely possible that GCW is just a "mean girl" by nature and she found some other mean girls and was in her element and they all had great fun messing with the OP. I don't get that sense, but of course it is possible. But that would be something OP could find out for herself with a private talk. Even if there isn't time for one before the actual party.
And I agree also with all of the other advice about "watch your back". Something weird went down here. It needs to be paid attention to. It isn't necessary to freak out and jump to any conclusions. But I think attention needs to be paid.