Apple announced it's added support for external GPUs to macOS and some Mac hardware in a new High Sierra 10.13.4 update, after promising the feature last year at WWDC 2017.
Apple said MacBook Pro notebooks released in 2016 and later, iMacs launched in 2017 and later, and iMac Pro will support some external GPUs it recommends. Apple aims to give VR creators and pros more graphics power.
The supported Macs need to have macOS High Sierra 10.13.4 installed, and be connected to one of the "proper" graphics cards:
As CNET points out, Apple isn't supporting Nvidia external GPUs and hasn't provided any clarity if it will. You'll also need to plug in an external monitor to see the benefit.
Apple writes on its support site:
It's important to use an eGPU with a recommended graphics card and Thunderbolt 3 chassis. And if you're using a MacBook Pro, the eGPU's Thunderbolt 3 chassis needs to provide sufficient power to run the graphics card while charging the computer. Check with the maker of the chassis to find out how much power it provides, and make sure that it's enough to charge your connected Mac notebook.
eGPU support in macOS High Sierra 10.13.4 is designed to accelerate Metal, OpenGL, and OpenCL applications that benefit from a powerful eGPU. However, not all applications support eGPU acceleration.
Perhaps external GPU on macOS support is paving the way for Apple's rumored 5K external Apple display with an internal GPU.