Using Java SDK, it does not appear that there is a way to manage Group Admin roles.
The API {{url}}/api/v1/groups/{{groupId}}/roles/{{roleId}}/targets/groups
Doesn’t appear to have a method call in the Java SDK 1.5.2 or 1.5.3.
Closest is User.listGroupTargetsForRole, but this only works if a user has already been assigned to that group.