After getting Giga studio finally installed, authorized, and working correctly, I discovered that sound is distored whenever Finale is open. The virtual keyboard plays fine with Finale closed. I tried using Cubasis also, I got the same result.
You've likely got two MIDI paths into Giga - one direct from the keyboard, the other from the keyboard, through your notator/sequencer then to Giga.
Click the little keyboard icon next to the midi port boxes in the lower right corner of Giga. That turns off the keyboard into Giga. Your midi will then only enter Giga through your other program. You can also launch the sequencer through Giga, and that turns off the keyboard input automatically.
I have exacly the same problem with Sibelius + GS3. My computer has 2GB Ram so I'm using tweak to be able to load more samples. When I change tweak settings I stared to get this problem. I'm not using any external midi devide at the moment. And also I'm using RME multiface. It could be RME driver problem or tweak thing.
What is your soundcard? Have you used any tweak for loading more samples?