The problem comes from mavros: Erle OS images are running the 0.18.3 version by default, which was released in July 2016. Many improvements have been made since then, but these new features are not implemented in the mavros version running on the Erle-Brain.
By having a closer look at the source code of the mavros "fake_gps" plugin, you will notice that it uses the ATT_POS_MOCAP message to encode the data:
Unfortunately, this MAVLink message is not supported in APM yet (only PX4). When looking at the source code of the AP_GPS_MAV library, which implements the support of a "MAVLink GPS", you can see that only GPS_INPUT and HIL_GPS messages are compatible:
I suggest you to have a look at the other topics I have opened on the subject:
Unfortunately, all this was a project for university and I had to return it recently. I could not achieve my goals in time, but I still hope someone will one day be able to inject an external position estimation into the Erle-Brain.