GigaStudio 3 and Sonar X2a

Been down this road (though it's been a while).  For converting/playing back giga format, here are my recommendations:
 
1) Kontakt.  Ain't cheap, but if you already have it, then it's a no-brainer.  Does a decent job importing and playing back giga files.
 
2) G-player.  A one-trick pony... Probably not worth its price if you don't have a substantial giga library, but if you do, this is the king.  Probably the most accurate because it plays giga natively -- there's no converting.  Another advantage is that it's still being developed and updated.
 
3) Samplelord.  An older, multiformat player, and stopped development at 32 bit.  Still, I used this for years before getting Kontakt.  In terms of accurate playing, IMO it's second only to G-player.  Sad it's not being developed anymore, because the dev really knew his stuff.  (This player is also excellent at playing soundfonts).
 
4) Linuxsampler.  Free, but rather complicated to figure out.  I never tried this one, but other users have sworn by it.  It's free, so if you have a small Giga library, and are tech-oriented, this one's definitely worth a try.
 
These all have demos, so you can try them out.