Icom Garmon NEMA communication
Check the version of NMEA that the Garmin is using. You should be able to chose between 2.0 and 1.5. If you look at the "Words" that the Garmin uses, they have better communications with the older level (1.5) than the newer level (2.0) You also need to look at the "Words" that the ICOM 502 can read.
By switching the NMEA on my Garmin to 1.5, I was able to get my GPS 128 to talk to my NEXUS network and my ICOM 502.