group snapshots are not listed in order of creation
When making a list of group snapshots the group snapshot id is used as an order key. A group snapshot id is generated by librbd on the snapshot creation  by a custom function  using a combination of the client instance id and a random number. So the ids are not monotonically growing, and when listing the snapshots the snapshot order is different than the creation order.
Thus there is no an easy way to determine the group snapshots order.