t's for power (the second PCIe connector). It doesn't have PCIe 8-pin but that proprietary PCIe power through the fingers. I guess it's a Mac exclusive.... that sits in its own enclosure (MPV Duo, for two of them). The first thing that popped up in t3h g00gl3.
What's the Thunderbolt 3 connector for? The PCIe 3.0 x16 connector doesn't supply enough bandwidth?! Could you use the Thunderbolt 3 connector for video output (there are adapters for Thunderbolt 3 to HDMI)? Could you theoretically use a Radeon Pro Vega II in crossfire for gaming?
The TB3 connector is for video. I believe that link even said as much.
As far as your question about gaming, it's a pro card, so, IIRC, drivers for those don't play well with games and I'm not sure you can install the game drivers on a pro card. In googling for you, I ran across an article that said don't expect to game on this card....... but I didn't click.
The Radeon Pro Vega II Duo has built in HDMI 2.0 according to the article, but why the 40 GiB/s bandwidth Thunderbolt 3 connector? Would high DPI and/or HDR video editing require that kind of bandwidth for a monitor?
Considering its proprietary PCIe power edge connector I guess it doesn't matter if it can game.
Why didn't AMD just go with three 8-pin PCIe power connectors rather than a custom PCIe power edge connector?