I think this is about acknowledging feelings. Divorce doesn't mean feelings no longer exist, it is a change of circumstances and each new change of circumstances may bring new feelings.
When my parents divorced, no other parties were involved, but when eventually they both started seeing someone else, both times it did feel like there was a shift in our family, even though they never met the new partners. One more step away for my parents from each other and another sign to us children our parents were moving in separate directions.
When my Mum decided to marry her new partner, I knew it would affect my Dad - and it did. Not in any bad way, but it was the sign that their marriage was truly over. They would never have reconciled, but until that time when one decided to remarry, it was like neither had made a commitment to another person. Each may well have found new partners, but marriage was a different story. I know it unsettled him and he was concerned enough to check out my prospective step-dad. He didn't know him, but they were both members of the same social club, so he could ask around about him.
Grannyben I think your feelings are understandable given your ex-husband started a relationship with someone else who is already part of your extended family, particularly as you'd become friends with the other GM. I don't know if you shared experiences of your marriages breaking down with the other GM when you were supporting her, but you may have shared things you now wish you hadn't.
It would have been nice if one or both (separately) had taken the time to tell you they were starting to see each other. But it sounds like that didn't happen and it might be that your children didn't know either, so everyone may be feeling a bit awkward right now. Or maybe they did know, either way they could be feeling awkward and unsettled.
What happens next really depends on whether or not you can get over how you feel right now towards the other GM. If she was a friend before, perhaps you could be again. You could also look at it this way - your ex-husband has chosen someone you like and is already part of the family. Imagine if he chose a stranger you or your family didn't like.
Your feelings are real and there's nothing wrong with them. You've shared them out loud on here. What happens next is what you do with them. I wish you well with your new GC and its new family. 