To generalize your question - the Gantt chart widget doesn’t seem to allow you to choose any column you want as your Group By column. It only allows you to select from a few default column options. Is that right?
I seem to be in a similar situation - I have a board with several groups representing the different teams working on a project with multiple phases. Each team has its own set of tasks that I follow up on with them. I included a additional custom column that specifies the project phase associated with a particular task. But in the Gantt view I can’t select that column to group by.
It’s strange because I can do that in the Main Table (it will create that “regrouped” table in a new view), but not in the Gantt view. Nor can I figure out how to create a Gantt view from the regrouped table.
In hindsight, I should have just set the groups up based on the project phases instead of the subteams, and then filter on the teams to see their separate tasks. But it would be a good idea to have the flexibility to group based on whatever column you want in the Gantt view.