Gaming on Apple is going to depend on how well optimized the drivers and shaders for the GPU are going to be. You'll also need some serious upscaling because most Macs come with super high res screens, a GPU that won't be able to drive modern games at those resolutions and an utterly lack of graphics card support, even in the new Pro.

Another factor is that major GPU companies put efforts into rewriting some games' shaders to make them run better on their hardware. I doubt Apple is going to do that so you'll probably end up with games performing better on other platforms soon after release.

There's more to getting games to run smoothly on a platform than compatibility software. So far, the GPU has been lacking despite Apple's attempts to hype up gaming for mac. With the rate things are going, the year of gaming on Mac may be the same as the mythical year of the Linux desktop: inching ever closer but never really arriving.