Well, I found this as an alternative to GS wavetable synth. It's a Timidity installer that DOES work on x64. Everything is in Japanese so I used google's translator:
Firs I got to this site:
TiMidity++ windows synthesizer(TWSYNTH) Project Top Page - SourceForge.JP
I downloaded and installed that. That thing added a driver which I can use in Guitar pro (and I suppose in other programs).
Then I updated the content of the Timidity folder in Program Files with this:
Google
That guy seems to have worked with Windows 7 x64 and updated Timidity's files.
It sounds similar to GS wavetable synth, with little differences For example, drums are panned on the listener's perspective and cymbals are a bit loud. Also I am not able to mute drum sounds anymore (as in when a drumer plays an open hat and then closes it with the pedal, you know), but that's not so terrible when composing