See this note in the GCM
architectural overview:
Note: If your organization has a firewall that
restricts the traffic to or from the Internet, you'll need to configure
it to allow connectivity with GCM. The ports to open are: 5228, 5229,
and 5230. GCM typically only uses 5228, but it sometimes uses 5229 and
5230.
GCM doesn't provide specific IPs. It changes IPs frequently. We
recommend against using ACLs but if you must use them, take a broad
approach such as the method suggested in
this support link.
Katie