What is the ICAO equivalent of /G in a flight plan?
I have a GNS-530W, which is identical to your unit except with a bigger screen and I file ICAO with a G, B, and S, which are GNSS/GPS, LPV, AND VOR/ILS, respectively, to indicate the equipment I have on board. Technically just the G would be sufficient but I list all I am capable of to assist ATC in routing me most effectively or in the event of an emergency.

This would be entered on the form as "G, B, S"

Here is an excellent article explaining the new fields and codes for filing ICAO as the FAA domestic flight plan form comes to an end soon:
Filing ICAO Flight Plans in ForeFlight