If what you've downloaded is a Gmax source file then it's a fairly straightforward job. If it's a model compiled and ready to fly in FS9 (which I suspect) then it's possible, but almost as much work as building a model from scratch.
You must ask permission of the author and if it's a compiled model ask if you can have the Gmax source file. Some modellers are very generous that way. One of our CFS3 modellers has quite a catalogue of aircraft originally modelled by others which he has adapted for CFS3 and released.