Yes you can do this with the Cubasis midi instruments and then use Audiobus to send the audio to GTL.
I wouldn't recommend using Cubasis as an effects processor for live instruments though if your planning to send them on to GTL or any other app. You can try it but you might run into some latency issues. Again the best way to add effects is to use Audiobus as it does some clever latency compensation for live instruments. With Audiobus you can add any effect app to the audio input, before it gets to GTL.
By the way in the next update of GTL, due soon, you won't need Audiobus to do this anymore. You will be able to add effects apps/audio units all from within GTL.
Also you might want to check out the Audiobus forum if you haven't already.
https://forum.audiob.us