Can the Garmin GI 106A be used with a Garmin GNC 300?
Yes, you can use the 106A for GPS right/left indications. As the 300 does not give glideslope you may want to use a GI 102A (no GS indicator).
Is the Garmin 106A CDI - Vor / Loc / GS fully functional with a Garmin 420 WAAS?
A 106A CDI - Vor/Loc/GS will work with a GNC 420W GPS but you are limited to only GPS and WAAS approaches. You would not have Nav VOR, ILS or glide slope capabilities.
Does the Garmin GI-106A CDI- VOR/LOC/GS work with the SL-30?
Yes, the GI-106A is commonly used with the SL-30.
will all functions of this Garmin GI 106A VOR/ILS/GPS Indicator work with the garmin GTN650 and is it a good match for IFR certification
Yes, The GTN 650 will support all functions of the GI 106A. Garmin recommends the GI 106A for IFR flight.