I was using the iPad version of GB as well. Right now the only option to get files from GB to Cubasis is to export your mix or export a solo track, then email it to your self. You could also copy it over to your desktop via iTunes the import it into Cubasis via iTunes but there’s some extra steps.
The only drawback to mailing the GB file to yourself is that it will be converted to a compressed m4a file.
Here’s a link to a YouTube file someone posted here a while ago showing the GB > mail > Cubasis workflow.
