I'm not entirely sure from your post what process you use to load instruments, but I use GIGA 3 and SX 3.
Here is how I load things in GIGA, and how I choose sounds within CUBASE. Hopefully this is helpful... [

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1) In GIGA I open up the Quicksound database at the bottom of the screen. There I navigate to the .gig file that I want (i.e. VI-14_BASIC_SET), then click the + sign to show each available articuation.
2) Drag and drop the articulation you want (i.e. VI-14B_stac) into one of the GIGA instrument channels above. If you want to use more than one articulation you can use a keyswitch version (i.e. VI-14B_basic_all) or load additional articulations into other instrument channels.
3) In CUBASE, you have a few options for accessing GIGA instruments. The easiest one is to use REWIRE. Enable GIGA's REWIRE channels 1&2. This will make all GIGA instrument channels appear as MIDI Output options in your CUBASE MIDI channels. You will see the names of the articulations you've loaded listed as options (as long as you're using GIGA 3.1 on your system, otherwise it just lists generic REWIRE MIDI channel #'s, not names). (This is down below the generic GIGA PORT 1 you mentioned in your post.)
4) You can also use the generic GIGA PORT 1 and then select the specific MIDI channel you want within the CUBASE MIDI channel.
5) You can also use the VSL MIDI ROUTER, then open the Performance Tool as a go-between for GIGA and CUBASE. This is a little more complicated, but will allow you to use the Performance instruments.
Hopefully this helps a little. If you have questions, please feel free to PM me. (I definitely should not write instruction manuals for a living.) [

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Cheers!
--Leo Wolfe