Well I can say that I haven't been able to use it with VE Pro yet as I was the one to turn it in for Martin and he got it fixed (however I can't confirm this yet as I was only told it was fixed, I haven't actually been able to test it). So once the update comes out I can tell you for sure how it works.
I am staring at my G-Player right now and going to try to answer some questions.
I actually don't use the VSL samples with G-Player. I have a ton of libraries for giga but never purchased the VSL libraries back then. My friend however does own the old giga opus 1. He brought over his firewire drive with those on it and they all worked perfectly from what I seen. We tested most every instrument in the library (loaded it up, and played it to hear if it popped or crackled etc. It played the proper dynamic and patch as well so if it was a crescendo violin patch, it played it perfectly). He also had a bunch of percussion that I don't own and all those worked. We tested every percussion track. The percussion was so good it made me realized that I need to buy the percussion library but I want MIR more at the moment I think lol. Unfortunately for me to test anything for you I would need to have him bring his stuff over to do so. But I can assure you the patches worked. I don't know anything about the performance tool though because I wasn't using the library back then, and I have no idea how it interacts or works with the samples.
I can say that it's far more stable then Gigastudio ever was though. Gigastudio was one of those things where you cringed when you started up the machine because you weren't sure if it was going to blue screen or if the machine would lock up and force you into Gigastudio Safemode. G-Player has never caused me any crashes or Logic lockups (other then the compatibility between it and VE Pro). When it first came out it had below acceptable playability with my samples. However, they continued to update it and I worked with support, sending in logs and issues and it quickly become able to play almost 100% of all my libraries. I think I have around 100GB of samples in giga format and there is only a few left that don't play accurately. (actually off the top of my head, the only one that I can recall was GigaPiano 2). I think the GPulse files were screwing it up because it's built into that sample. Whatever they have been doing in updates has worked. It's a very simple player but hey it lets me use all my giga samples again. It doesn't have the full potential of Gigastudio, but it does what I need it to do and doesn't crash Logic. (It works perfectly in Mainstage as well and doesn't hog up my resources).
But honestly I could care less about that piano sample since I use Pianoteq now. I do however WISH that Pianoteq sounded as good as my Giga PMI Bosendorfer 290. Which by the way plays perfectly in G-Player. I have a lot of garritan products (harp etc), I have the Notre Dame de Budapest organ which sounds incredible and works excellent. To many to list here but the point is they all work fine. My beloved Atema's Harpsichord and my Flemish Harpsochord work flawlessly too which makes me happy. Oh and I do know we tested the old giga VSL Harp library and it worked perfectly as well. The keyswitching in the program works and is very simple to use.
Half of my giga samples now a days are pointless because I have replaced them with better VSL patches but my harpsichords, guitars, voice of the apocalypse etc that I have for giga are still very good. I also have some rare instruments that VSL doesn't make and it's nice to be able to mix those in my projects as well.
The G-Player does a stereo or multi-timbral player, or a solo g-player within your DAW. It already uses little resources but then being able to freeze the tracks just makes it that much more useable and resource friendly. Even if your giga sample wasn't programmed for key switching, you can initiate a stereo version of the player, and then load up to 16 articulations into that window and simply send midi channel changes to change patches. It's worked fine for me that way.