Oh, mom's groups. I'm surprised there isn't a reality show.
So, here's what I would say first. There is a good deal of research that suggests that parents of autistic children often fall on the spectrum themselves. (Please moms of autistic kids, don't message me). It's true! And some of the things you described sound a little like Asperger's:
Here's an excerpt from WebMD's definition of Asperger's:
Not pick up on social cues and may lack inborn social skills, such as being able to read others' body language, start or maintain a conversation, and take turns talking.
Talk a lot, usually about a favorite subject. One-sided conversations are common. Internal thoughts are often verbalized.
Ok, so there's like TWO symptoms out of their list but it describes pretty well what you are talking about, I think?
If that can help you understand her, or have empathy for her...and maybe her husband is this way as well. Maybe her husband has other deficits that are different than hers and between the two of them they passed on all of their spectrum issues to their kiddo who is autistic.
But, really whatever the cause, here's what I think and how I feel.
People are in our lives for a reason. Sorry to get all meta on you here but I think we come across people in our lives at certain time for certain reasons and the ones we have the most difficulty with can teach us a great deal about ourselves.
Clearly, you want to do the right thing but you also want to have appropriate boundaries and I think that's great.
You need to think about this situation not in terms of "us" (you and the other moms) but in terms of YOU, how YOU feel.
Sometimes being there for someone else and doing the right thing by someone else is good for our soul. You spent a whole summer with her and things worked out. Then, something shifted and it sounds like there's a little too much Gina in your life, so you need to pull back.
I would say that you stop the MNOs altogether for a little while--through the summer, maybe. Just do playdates, attend to other friendships and let Gina and everyone else know the MNOs just aren't going to work out for now. You're exhausted, you've got TONS and TONS of plans this summer. Hopefully she'll find something else to do and you can just kind of go your separate ways when it comes to socializing in that regard.
With the other moms, pick one and go to a movie one evening. Pick another and go to a book reading or a coffeehouse or to get a pedicure. Don't talk about Gina, don't say anything about the situation.
In the fall, you can resume MNO quietly and when you aren't in the same class and then you have a reason for it to just be you gals, it hopefully will avoid hurt feelings, and you can do a good thing for her and her kid/kids by doing playdates once in awhile while you taper off the "togetherness" but, do it with compassion and you will be better for it.
I really don't think talking to her about it is a good idea, as other moms have suggested. If you don't think she'd get it, it's probably because she won't. Most people have the ability to self regulate in social situations, if she doesn't then she needs more intervention than you can give her.