It works, but not half as good as an equally priced nVidia card. But you don't have that option, do you?
In addition to what cc_merchant stated, you cannot purchase an iMac with both a GeForce GPU and a Retina 5K display in the same unit. I've just checked, and the only screens that are available with a GeForce GPU are a 27" display with ony a 2560 x 1440 resolution (non-Retina) or a 21.5" display with a 1920 x 1080 resolution. What's more, oddly you can choose an i7 CPU only in the 21.5" model; but the 27" non-Retina model is only available with an i5.