Same problem here. There is an issue with Cubase 4 in which an .xml file has to be edited so that GP presents itself as an "FX". By editing this file, Cubase sees it.
Best I can tell, the issue is the same in V-stack. The symptoms are identical (at least for me), in that Gigapulse does get registered, and shows up properly in the plug in info window. However, it does NOT show up in the list of available send effects rendering it unusable.
The bigger problem is that, as far as I can tell, there is no file to edit in V-stack. The Vst2xPlugins.xml file that is edited in C4 to make GP work properly is nowhere to be found in V-stack. At least I cannot find it.
Anyone have any ideas for a workaround? I hope that Tascam can fix this in a future update so that it properly identifies itself as an "FX' in Steinberg's products.