GPS: Under the 'comm' tab set the correct port to NMEAIn/NMEAout. Under advanced, settings should be off, off, off, 4 digits, names.
I think theres also a NMEAout menu option on the radio that might need toggling.
Cables for Com1
Garmin Tx1 (Blue) -> ICOM NMEAOUT+
Garmin Rx1 (Brown) -> ICOM NMEAIN+
or for Com2...
Garmin Tx2 (Green) -> ICOM NMEAOUT+
Garmin Rx2 (White) -> ICOM NMEAIN+
ICOM NMEAOUT- and NMEAOUT- should be connected to the same ground the gps is connected to. You'll have to look at the diagram in the manual