I can't see any reason that GVI and GS3 won't work on the same machine. Obviously you can't load a GVI instance into GS3 as GS3 doesn't support VST instruments, just VST effects. However, both should be able to load each others samples. In fact, some of the samples delivered with GVI are exactly the same samples in GS3.
I not sure you would want to use both at the same time as they would compete heavily for processing resources. I haven't tried that yet, as I don't have a reason to do so. For live playing I would use GS3 and for multitrack work I would use GVI.
The difference between GVI and GS3 are centred on the fact that GVI is a VST instrument plugin. Therefore GVI has no mixer or editor, other than the Quick editor. GVI is more suited to DAW applications such as Sonar and Cubase where you can use it as a synth for a midi track. With GS3 you would either have to use GS3 in rewire mode to insert it as a synth otherwise you would have to run GS3 outside your DAW app and somehow reroute it back into DAW via your sound card. In rewire mode GS3 only seems to support one port and Sonar only supports one instance of rewire and hence I only get to load 16 instruments. In addition GS3 in rewire supports only one stereo out channel which is rather limiting. With GVI, on the otherhand, you can load multiple instances into your DAW for unlimited instruments and as many outs as you have instances. I'm a little disappointed that the GVI manual recommends only using one instance and loading all your instruments in the one instance, which sort of suggests its no better than using GS3 in rewire mode.
Another advantage of using a VST is that the DAW app is aware of the characteristics of the plugin and can automatically compensate for any latency of the plugin.
I think you can load all your GVI sounds into GS3. I haven't loaded them all but the ones I've tried did work.
For DAW I run Performer on a Mac so at the moment I can't run GVI as a plug-in. I have run GS3 on a Windows machine for years and I am thinking of running GVI as stand alone on this same Windows machine or another Windows machine.
How about the actual sounds and synths.?
Is it true that GVI is a soft synth and a sampler?
Where as GS3 is just a sampler?
Does this mean that GVI has synths. and samplers?
I am a bit confused as to what actual sounds GVI will make that GS3 cannot make?
I can't see anything in GVI that could make it a softsynth rather than just a sample player. I seems to have the same functionality as GS3. With GS3 you can use the Quickedit functions on a sample to morph the sound like a subtractive synth would. GVI has much the same ability. They both have amplitude, filter and pitch modulation envelopes and LFOs.