Alright. Wine is seriously cool! The only problem is that it takes a LOT of time to compile and it took out nearly 400MB from my /usr!!
Anyway, its way too cool to cry about all that.
The thing is, Guitar Pro 5 works great in wine. All I needed to do was to install timidity++. I had to go ~amd64 on it, because the current stable 2.13.2 didn't compile. I was not in a mood to debug it, so I just changed the keywords and the testing version 2.13.2-r2 worked perfectly. I went with the default timidity-eawpatches.
Next came wine. It failed during ./configure because /usr/X11R6/lib/libGL.a existed, which, from what the error message said, prevented linking against the OpenGL libs. I thought of going -opengl for wine, but then felt like experimenting and moved the libGL.a file somewhere else. libGL.la exists in the same location anyway. Wine got installed fine then.
Once, done with winecfg, I launched the Guitar Pro installer with winelauncher. It installed fine, even with that RSE thing selected.
With RSE however, it doesn't play the sound properly. I had to remove RSE and it worked perfectly. Also, do not touch the Change Skin button in GP5. Trashed mine. Plus of course, the Windows Volume Control button doesn't work, which is kinda obvious.
I have slight performance problems when a KDE notification pops up while playing a file in GP5. But that's a minor. Maybe I wouldn't have the problem if I disabled XGL. Who knows?
Anyway, so there I am. Another step closer to "Bye bye Windows!". The only thing that remains now, is running Pro Evolution Soccer on Linux and this will be a single boot box of Gentoo! :D