Bob,
I remember reading when the Gwire unit first came out, that someone took the signal off of it and hooked it up to a Digitrax or similiar decoder.
The signal coming out of the Gwire is probably just the 5v DCC signal (or some version of a DCC signal), not what you would normally see on the track of a dcc system (DCC signal on top of square AC wave or similiar), which is why you would need to build a booster here. However, you can probably take this 5v DCC and send it directly to the uProc pin on the decoder.
I did something similiar last year as an experiment with a different DCC system and a function only decoder. Basically I sent the DCC signal from an NCE command station over RF. Then captured the signal with the reciever RX, and sent this digital signal directly into the uProc of a function only decoder. I had to hack this decoder. I forget the specifics, but basically somewhere in or right after the diode bridge rectifier on the decoder, a line comes off and goes through a resistor setup or two (think it is a voltage divider setup to convert to the 0/5v DCC signal) and directly into the uProc. This line is filtering out the DCC signal, droping the voltage so the uProc can handle it, and sending it to the uProc. So you need to cut out and isolate this line and feed it from RF reciever or possibly Gwire unit.
I was going to try this with a motor decoder but never got around to it. Note that motor decoders may be a little more complex, in that it will take the DCC signal from both sides of the rail and feed it into 2 different pins on the uProc. The signals on each side of the rail I believe are bi-polar or opposite of each other, and I beileve many motor decoders compare these signals as a safety check to ensure its getting a clean and accurate signal. I think the way to get around this is to branch the single signal and send one side through an inverter, but not sure.
-Ray