You can create a milestone for a group that will be shared across group projects.
On the group's **Issues ➔ Milestones** page, you will be able to see the state
of that milestone and the issues/merge requests count that it shares across the group projects. To create a new milestone click the **New milestone** button. The form is the same as when creating a milestone for a specific project which you can find in the previous item.
In addition to that you will be able to filter issues or merge requests by group milestones in all projects that belongs to the milestone group.
Project milestones can be promoted to group milestones if its project belongs to a group. When a milestone is promoted all other milestones across the group projects with the same title will be merged into it, which means all milestone's children like issues, merge requests and boards will be moved into the new promoted milestone.
The promote button can be found in the milestone view or milestones list.