I have implemented GCM using my own sever. Now I'm trying to do the same using Python 2.7 in Google App Engine. How can I get the IP address for the server hosting my app? (I need it for API Key). Is IP-LookUp only option? And if I do so will the IP address remain constant?
You can check the IP easily by doing a ping from the command line to the domain name, as in "ping appspot.com". With this you will obtain the response from the real IP. Unfortunately this IP will change over time and won't make your GCM service work. 
In order to make it work you only need to leave the allowed IPs field blank.
API Key for GCM from GAE