I personally doubt it's possible.
EFI needs an ECU (Engine Control Unit) as it isn't just an electric motor sucking fuel from the fuel tank and pumping in the combustion chamber. It's much more complex than that. And I'm not entire sure, but I think The GS is an old school CDI bike and doesn't possess and ECU!
Now, what ECU is?
Think of The ECU as a computer that monitors and optimized an engine's performance by data it receives from various sensors located around the bike. It monitors wheel's RPM, engine's RPM, heat emission, temperature, combustion, ignition timing and calculates the optimal fuel/air mixture given the outside temperature, moisture and even elevation. And only then the fuel is pumped into the chamber according to all this data. There's a complex computer algorithm involved.
You don't get all that efficiency just like that! Besides, a fuel pump can't possible know how much fuel to pump in the chamber! It'll just keep pumping the same amount of fuel regardless of the weather and engine's condition. And this means your bike will either be running too lean or too rich in most scenarios!
Now, I understand that the GS is notorious for being too 'cold blooded'. Needs a lot of choke to get the engine started in winter which puts a lot of strain on the ignition coils and the battery but...
You've to live with that!
Try to park your ride inside the house where it's nice and warm. And if that's not possible then...
Well, then you're out of luck!
P.S I'm not an engine expert. So don't take my advice to the tee!
