The new C++ API allows to implement other backends than the filesystem.
There is a mock-up implementation for this already.
In order to implement actually implement it, the functionality in the gcs namespace needs to be implemented using the google cloud sdk. For reference, see the implementation in namespace filesystem.

The steps involved should be analog to #136. I would prefer to have the aws implementation first, but if someone wants to take a shot at implementing the gcs binding this would be very welcome too.