If you mean the one here
http://www8.garmin.com/support/commProtocol.html
This is not what I would term an SDK. It is a trivial code sample demonstrating how to read and write USB packets to a Garmin USB GPS driver under Windows. If you are going to use this on a device it is not relevant as Garmin only seem to do a USB driver for the desktop Windows OSs.
Assuming that you will be using an RS232 interfaced GPS talking to a PDA. I have looked at the Interface Specification pdf included in the "SDK" and you may have some "interesting" format conversions to do as some of the data sent to the GPS is effectively a memory map of the data. You won't be able to easily do this within B4PPC and will likely need a custom library to produce the data packet.
Please don't take this the wrong way, but the words "aircraft", "navigation", "cockpit" (and "crashes"!) make me shiver. If what I have said above is not already obvious to you are you sure you have the technical skills to undertake this?