You might wish to take a look at the µg (microG) project. It's currently between its alpha and beta state, but I'm using its predecessor (NOGAPPS) for quite a while and must say it works pretty well. And despite its early state, reports in the linked XDA thread sound real good: GCM seems to be working, and even the Playstore app accepts the backend.
The minimum configuration is described there: µG GoogleCore and the proxy should do – together with the Fakestore (a playstore stub with no functionality but letting apps believe it's there) for an absolute minimum should do. If you can live without the Drive, Fitness, and Games API, and without G+, it's the way to go. Currently the dev is working to add Wear support, and get the other stuff out of its "baby shoes".
Necessary .apk
files can be downloaded from the first post of the linked thread.