New iMac , new GPU

Just bought the iMac with the Radeon Pro 560 4096 mb GPU. If I’m using the cycles renderer and I’m using rendered viewport shading mode option, is it the GPU that is working or the CPU ? My former iMac used cuda and OpenGL (gforce card) but now I only have an OpenCl option in the user prefs though in the properties panel there’s an option to use GPU compute. Radeon is al kind of new to me, I’m also wandering if I need to install aditionnel drivers (like cuda for nVidia).
And what has Apple Metal to do with this GPU ?

If you have “GPU compute” active (and it’s not greyed out) in the renderer properties, then it should be using the GPU. You don’t need to install anything else.

Apple Metal is an Apple-specific graphics API (something like OpenGL), but Blender does not use it.