It depends on what options and filters are being used in the profile you’re using. Not all adapters support all options. For example if the profile is using a denoise filter to clean up artifacts and if you’re hardware doesn’t support it then it’ll use the CPU to do that part. If you want to use only the GPU then you’ll have to build a custom profile specific to your GPU capabilities only and remove all other options.
MCEBuddy uses hardware acceleration to assist and speed up encoding without compromising quality (to the extent possible - CPU encoding will always have higher quality than GPU encoding for the same bitrate).