The differences are a lot greater than simply clock speed. The GPU is designed to do a few things well, whereas the CPU can do a lot more.
This analogy is quite good, taken from
here
One way to visualize it is a CPU works like a small group of very smart people who can quickly do any task given to them. A GPU is a large group of relatively dumb people who aren't individually very fast or smart, but who can be trained to do repetitive tasks, and collectively can be more productive just due to the sheer number of people.