Giga is not a VST instrument so it won\'t work to rout it to VSTack. It will work as a rewire instrument and FX-Teleport is working on a VST wrapper so that Giga can be seen as if it is a VST instrument. I\'m not sure if that would work in V-Stack but at that point, you would just use FX-Teleport. FX-T is an alternative to V-Stack.
Also, check into Bidule, a program that does handle VST and Rewire instruments together. That is a possibility. No garantees on how it will all work together.
Hopefully, the eventual solution will be for Giga 3.x to host VST instruments the way it will current be hosting VST effects. That would rock.