Hi Matthias,
at the moment, we don't have an endpoint that can create new group policies.
You can create a network and then manually create the group policy in that network, and then via the API create a new network based on the one that has the group policy already in it. But as you want to create/update group policies on existing networks, this is not currently possible.
I believe we are currently looking in to adding this functionality, but I'm not sure o timescales, etc.
Thanks,
John