This can happen when yoiu install another application that GigaStudio reads as a VST plugin. Try this:

1. Close all applications.
2. Start GigaStudio.
3. Start Pro Tools
4. Open a different ReWire application so that ReWire reinitializes.
5. Close both applications.
6. Restart your computer.
7. Start Pro Tools. GigaStudio should now show up as a valid ReWire instrument.