The question is why would you need to implement both. If you only need cloud to device messaging, the GCM HTTP solution is much simpler to implement. If you need device to cloud messaging too, you must implement connection to GCM CCS, in which case you'll have both device to cloud and cloud to device messaging (and wouldn't need the HTTP solution).
That said, there shouldn't be any problem implementing both at the same server. For sending messages to the GCM HTTP endpoint you'll need to send HTTP requests to that endpoint. For sending and receiving messages to/from the GCM CCS endpoint, you'll have to establish a XMPP connection. Both can be done at the same server.