are you trying to get the iPhone to use the gps signal in your garmin receiver to control the google maps application on your iPhone? well, i hate to break it to you, but that is not possible. there is nothing in the iPhone's current software that will allow it to recognize a gps device via bluetooth and use the gps signal to interface with google maps. no way around that, unless you happen to be a spectacular software hacker who can take the iPhone's software and rewrite it to do this. you'd need to adjust the bluetooth interface to allow it to receive gps data, and re-write the google maps application to allow it to take gps data and utilize it. and i'm probably over-simplifying what would need to change since i'm not a software engineer myself.
i hope you didn't buy the iPhone thinking this would work. if so, what led you to believe this? gps capability (stand-alone or through google maps) was one of the most common questions that came up since it was announced in january, and the answer was consistently no from Apple and david pogue and other journalists who had access to the prototype iPhone in january and before it was released in june...