@martinfilliau all "unavailable" devices are retried. This is determined by if either the GCM reports them as "Unavailable" or there is an InternalServerError with GCM. Per:

https://bitbucket.org/sardarnl/gcm-client/src/a8bf40afcfd9e580b02b03d5c247bf1e05d53631/gcmclient/gcm.py?at=default#cl-170