You can mine Gridcoin with a GPU as well, but it is not as profitable as many other coins. Those who do mine GRC with GPUs do so mainly to contribute to the scientific endeavours.
GRC is one of the, or maybe even the, most profitable coin to mine with a CPU. Further, CPU miners only compete with other CPU miners, and GPU miners only with other GPUs (generally speaking).
So it's actually better to mine Gridcoin with CPU than a GPU, if you are looking at it as profitable?
If you are looking at the best return, then yes. Assuming you do a good job on project selection, you will earn much more with a CPU than a GPU per Watt of power you invest.
Of course, a GPU will get a lot more work done. This is important if you are trying to make a big discovery with your machines. The more work your machine completes, the more likely it is that you will make a big discovery and get published in an academic journal.
That is the whole gimmick of GPU's - they are much faster, but much 'dumber' than CPUs. Therefore, they are good at running simple jobs really fast, such as decoding Enigma messages.