Also be sure to turn on the NMEA output on the Garmin unit.
Out of the box the unit will be set to send "Garmin" and not NMEA.
You will need to be sure that if it is an Icom set that you have a common earth for both the radio and the GPS.
With regard to displaying "Distress" and "Position Resquest" calls, not all Icom units send this info.
Only the M505 and M603 in the current line up do.
This function will work on all Garmin and or Standard Horizon plotters.
BTW all the current SH radio's send this info.