The stock FS2004 GPS does not model the Garmin GNS series very well. There are some third-party add-ons from
http://www.reality-xp.com that do a very good job. These need to be integrated into existing planes manually. It's straightforward to do but requires some editing of files.
If you would prefer a plane with RealityXP GNS packages already integrated, the Dreamfleet Beechcraft A36 Bonanza comes with a GNS430 and the Dreamfleet Beechcraft B58 Baron has a GNS530. The Dreamfleet site is at
http://www.dreamfleet2000.com