GigaStudio and Kontakt Player will compete for the same drivers on the same computer. Here's what you need to do to get around that:

WORKAROUND: After you've installed GigaStudio, run it before running Cubase and disable outputs 1,2 in Giga (Cubase grabs these outs so if you leave them enabled it'll cause problems). Close Giga and run SX and disable whatever Giga outs you are using (3, 4?) so that each output pair is only assigned to one app. Now close Cubase and run Giga again, go to 'Setting Hardware' and set all midi inputs to 'None' then go to 'Settings General' and set Sequencer priority to about 5 and click apply, (you'll need to experiment with this). Now run Cubase, you should find 4 Giga midi ports available in SX and that should be it. If possible, use 3 HD's, 1 for system, 1 for Gigs and 1 for audio. Recording Giga back into Cubase depends on your audio card and you may need to wire outs 3, 4 to ins 1,2 or something. Otherwise you could use the Giga record feature and import the waves into SX.

On the RME Site there is more technical information about using GSIF and ASIO simultaneously.