The stock pinion included with the GD600 is something like 2.5:1. Great Planes sells several optional pinions for this gearbox so you can get as little as 1.7:1 and as much as 3.8:1, IIRC. Optional pinions from third-party vendors will net you as much as 4.6:1. The gears are a standard "45 pitch" so any pinion with that pitch will work.

If you want to use the 2.5:1, you will need a different, smaller prop or you will overload the motor and burn it up on the first flight. What size prop that is, I don't know, but I do know that the 12x8 and 3.0:1 is an optimum combination for this plane.

You may as well leave the landing gear completely off the plane if you're not going to extend it so the prop will clear the ground. Then a folding prop will be a good choice. Otherwise, just get yourself a piece of piano wire a.k.a. music wire the same diameter as what came on the plane, and bend up some longer legs. It's not difficult, and is nothing to fear. Total cost of materials is a couple bucks.