All examples inside the Starter Bundle can be executed on a CPU without a problem. The same is true for all examples in the Practitioner Bundle, although some examples will take longer to run. In either case, a GPU will dramatically speed up the network training process but is not a requirement.
That said, if you intend on going through the ImageNet Bundle, you are expected to have a GPU with at least 6GB of memory. The more GPUs you have available, the better. You should also have at least 1-2TB of free space on your machine. The ImageNet Bundle covers very advanced deep learning techniques on massive datasets, so make sure you make the necessary hardware preparations.