Hi,
I had a similar problem with my setup - 4iiii crank-based power meter and Garmin's combined speed and cadence meter. GC was not displaying the cadence from the power meter, but instead using the Garmin speed and cadence meter (without a magnet on the crank it was showing zero cadence).
Reading through the code, I realised the Garmin speed and cadence meter should be set as a 's' (speed only) not 'd' (speed and cadence) type device in the configglobal-trainmode.ini, so that only the speed is used from this device, otherwise this device's cadence reading will be used instead of the power meter
Regards
Julian