Update: I picked mine up today! I can also confirm my previous impression that this is basically Goyard's version of the Victorine. Here are some photos and a comparison:
Dimensions: Practically identical; the Saint-Gabriel is just a *smidge* taller, like a millimeter or two. I forgot to compare the length with them fully unfolded, but I don't think there is a significant difference.
Functionality: I initially thought the SG would have an extra pocket compared to the Victorine, but didn't realize (though I see now that it's shown in the photos, duh) that it has a flat pocket behind the card slots only on one side, whereas the Victorine has two. Both have one bill pocket, one coin pocket, and 3 card slots on either side, with a snap closure. So same exact number of pockets and compartments, the question is just whether you want one of your flat pockets on the outside or both on the inside. Also, the Victorine has a leather-trim zipper pull on the coin pocket, whereas the SG is silver-tone metal. Otherwise, the exact same.
Pricing: The Victorine starts at $575 USD (pre-tax) for a coated canvas version with grained leather interior and trim; about $700-800 for full leather, and $900+ for higher-end seasonal pieces. The Saint-Gabriel, which is Goyardine with Vauzelles calfskin trim/interior and yellow goatskin lining, will run you $860 USD pre-tax.
Quality: I have the Victorine in monogram canvas, and I can tell you that the leather trim feels cheap as hell. Kind of looks that way, too, and the card slots are annoyingly stiff at first, to the point that it hurts your fingers to pull cards out. I know in the past there had been glazing issues with this style, but after about 6 months of daily use, my glazing is still intact. But in general, LV canvas SLGs are nearly indestructible and wear very well--my Victorine still looks just about good as new, and my previous daily driver wallet, the Sarah, still looks barely used after 2-3 years of consistent use. The SG? I don't really use Goyard SLGs, so don't know how the Goyardine can be expected to wear being jumbled around inside a bag. However, I do feel like the leather trim might be prone to scuffs and scratches; I thought it had a mark on it right out of the box, but was able to buff it out with a fingertip. The stitching on mine is kind of wonky in places also, which I don't love.
So is the Saint-Gabriel worth the higher price? I don't really know. It's hard to make a truly fair comparison without a few months' use, which obviously is not possible right now since it just came out. But based on my initial impressions, I think you'll get more bang for your buck with the Victorine, especially if you choose a coated canvas version. Victorines are obviously much easier to get your hands on, too. But it also depends on your taste, because if you really hate the LV logos and are willing to pay more to avoid them, the SG should function the same with a more discreet look, plus you get a bit more leather trim. Also, given that you're taking the time to read this in the Goyard forum on TPF, you're probably already a Goyard fan, so it might be worth it to you. And if you really need an exterior flat pocket--the lack of which on the Victorine is kind of annoying--for something you can easily access without opening the whole thing, the SG will be the winner.
In short, if you already have a Victorine, you really do not need to immediately run out and buy a Saint-Gabriel, even though that is literally exactly what I did because I am a dedicated Goyard fangirl lol. If you are in the market for a trifold wallet, the Victorine is going to do what you need it to do at a better price point, but even though I'm not sure that the SG is "worth" the extra cash, I don't think you would necessarily be disappointed with it as long as you don't mind paying more.
I hope we will see updates on wear and tear as more people get their hands on their own Saint-Gabriel!
