I'm seeing a lot of talk about it in this thread, but I doubt it's the W series. It's purpose is to accelerate women up the professional ladder to get more in top seats, and as far as I understand hardly functions differently and uses the same equipment as any other F3 series. It'd be more likely for GT to sponsor that series and not the other way around. Unless they do some sort of female-only series, but that doesn't seem like something W-series would need to be a part of, and sponsoring GT could be a drain of their resources.
Given that there's supposed to be a former F1 star at the event, I wouldn't be surprised if they're involved with the partnership in some way. I'm thinking it may not be Alonso and/or whatever he's currently partnered with, since he's to be the star of the new upcoming GRID game.
I'm leaning most towards Formula E, it's full of former F1 drivers, and the event is being held where the series concludes.
There's also Jenson Button in Super GT, which the GT franchise has always been close with.
Red Bull is a good guess too, with still ongoing developments of the Newey-designed X1 race car, usage of the RBR hangar for live events, and sponsorship of Newey's son's Super Formula car. There's Webber and Coulthard as potential F1 star material there, too. This could mean an RB F1 car or maybe eventually a Le Mans/WEC-spec Valkyrie if so.
It could be something smaller or less interesting (to us common folk, anyways) like the TAG Heuer partnership, being unrelated to the guest list and having little to do with new content. It could also be somewhere in the middle, like a new brand getting involved for special sport mode competitions/championships (like the Supra Cup), which could mean an influx of cars from said brand. Chevy has been close with PD historically with the C7 unveil through GT5, and would be timed well with the C8. McLaren have been delving into esports, and they could use GT Sport either as another extension of their Shadow project, or even just a standalone thing. Hamilton's involvement could mean some motion from Mercedes, though I don't know what it'd be. Maybe Lotus could make it's grand re-entry? Can't think of any other brands that stand out or are otherwise unoccupied/tied to another franchise/esports series.