At the moment you cannot specify the sorting logic within the groups in the Gantt, I’m afraid.
The order of the groups themselves should reflect the order of the groups in the Main Table view, if you’re grouping by Group. Within each group the items should be sorted by date automatically. This means that if one user changes the date of an item, it could change it’s position within the group in the Gantt view.
I can definitely appreciate how this would be a useful addition to the platform for yourself, and I’m sure for other teams too, so I would highly recommend opening a thread in our Feature Request section.
Here other users who would also like this added to the platform will be able to upvote the feature!
You can find this here: Submit an Idea - monday Community Forum
Best,
Dani