You can use a Gpu with the cpu portion of the apu, you see an apu is a cpu with graphics built into it. If you plug in a graphics card you wouldn't want/need to use the built in graphics as the card would be way better. The cpu would still work 100%
You wouldn't want to use the APU (iGPU) if you had a dedicated GPU. So yes you can use the CPU (just not the graphics portion, unless they allowed crossfireX which I don't recommend) if you have a dedicated GPU.
You can use a Gpu with the cpu portion of the apu, you see an apu is a cpu with graphics built into it. If you plug in a graphics card you wouldn't want/need to use the built in graphics as the card would be way better. The cpu would still work 100%