A Google Account used to be mandatory for GCM registration on pre 4.0.4 Android versions. That is not the case anymore.
If your app uses Google Play Services to register to GCM, you no longer need a Google Account, regardless of the Android version. If you are using the old client library (which contains the GCMRegistrar class), you still need a Google Account.
Here's what an Google developer wrote about it (taken from here) :
Some background: Froyo and Gingerbread registration is implemented in
GoogleServicesFramework, using the Google account for registration.
This has resulted in a lot of auth errors for people where the account
was not in a good state.
Starting with ICS, GCM doesn't depend or uses the Google account - you
can use it before you add an account or without any accounts.
The "Play Services" update is implementing the new scheme on all
devices - but it seems a small number of devices have problems with
this, we're investigating - but the numbers are far lower than those
with the old scheme.
If you want to use the code in GSF, for Froyo and Gingerbread - you
need to use the previous library, which sets package name explicitly.
The new library in GCM is using the new registration code.
The actual connection to google is following the same path - we're
gradually (and slowly) moving devices to the new code in play
services.