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From what I have read so far about GCM, yes it is very possible. The Google servers are in charge of talking to the android app running on your users device, so no problems there.
The heroku hosted GCM XMPP Server is in charge of talking to the google servers. Inbound connections are limited to ports 80 and 443 on Heroku so as long as you only need to initiate and connect outbound from a heroku hosted python app to the google servers then you are fine.
From the Heroku Docs..
Inbound requests are received by a load balancer that offers HTTP and
SSL termination. From here they are passed directly to a set of
routers.The routers are responsible for determining the location of your
application’s web dynos and forwarding the HTTP request to one of
these dynos.
I am happy to set up a test environment for you if you like to prove it.
You can also run a bash prompt to test things on heroku. just run :/repofolder$ heroku run bash from your prompt.
Here is a screenshot of heroku bash connected to gcm via port 5235
