Is using GPU instead of CPU "even if it is slower" possible?

i understand that the GPU and the CPU are different. but whenever this question is asked the answer is the GPU is "many slower processors" while the CPU "fewer faster processors", does that mean that the GPU is capable of doing the CPUs work but slower?!?!

and my second question is would a CPU with 100-4GHz cores be faster and better for graphics rendering(and other GPU tasks) than a 100-1GHz cuda core GPU?