I think no response is best. This way, the deluded ones who think you may actually change your mind and do this, are left wondering whether or not you will. When they don't hear from you, how long will it be before one of them GZ, or her sis, actually decide to re-open this and ask you again! Probably not the sis because she does seem to get what's been going on.
GZ has not apologised for hurting your feelings by not inviting you after her demand that you save that date. I mean what the fuck were you supposed to be saving the date for? Neither has she apologised for being so crass and rude as to ask you to do her decorations for the wedding she doesn't think you a close enough friend to invite to!
It will be interesting if she is so thick, and so insensitive and so bloody self centred that she actually request this again. Because she'll need to know whether or not you are agreeing to do this. If she doesn't get a move on her wedding will be undecorated.
No response stops you from having to be involved any further, and it remains to be seen whether GZ is finally able to take 'no' for an answer. (I suspect she's not, and that you'll hear from her with another plea)
After her behaviour, she at least needs to stew for a bit wondering who is going to be blowing up her balloons or whatever...