If your organization has a firewall that restricts internet traffic then you'll need to configure it to allow connectivity with Google Cloud Messaging (GCM).
The ports to open are:
GCM typically only uses 5228, but sometimes uses 5229 and 5230. GCM doesn't provide specific IPs; it frequently changes IPs. We recommend against using ACLs.
Meanwhile, the answer for that is 'all IP blocks listed in Google's ASN of 15169'. You can use an online tool to get the ASN info.