Gaming has never been a big focus for Apple, at least when it comes to its laptops and desktops, but with the growing hype around VR and AR the company wants to make sure it isn't left behind. In addition to the Valve partnership, Apple announced it is also selling an external graphics enclosure to developers who want to work on graphically intensive VR and 3D applications and games.
As the name suggests, Apple's External Graphics Development Kit isn't meant for consumers, but developers who wish to start optimizing their apps for external graphics processors with macOS High Sierra. A consumer focused external GPU isn't likely to arrive until spring of 2018.
The enclosure isn't built by Apple itself but a company called Sonnet. Priced at $599, it includes a 350W power supply, an AMD Radeon RX 580 8GB graphics card, a Belkin USB-C to 4-port USB-A hub, and a promo code for $100 towards the purchase of a HTC Vive VR headset. Apple notes that the HTC Vive promo codes have limited availability and are distributed on a first-come, first-served basis.
Only members of the Apple Developer Program to be eligible to purchase the kit directly from Apple's website. A Mac with Thunderbolt 3 running the latest beta version of macOS High Sierra is also required.