I have a GPU, but don't want BOINC to use it
There are times when you do not want BOINC to use the GPU. Using the GPU while you work, game or generally surf the internet can cause your computer to slow down. Although nowadays computers are more powerful and the newer GPUs have many gigabytes of memory onboard, so the load intensity will be less with these new beasts. However, if you just not want BOINC to use your GPU, that's possible.
Do you not want BOINC to use the GPU while you are at the computer, but is it fine for it to use it at other times? In that case, you want to opt for the preference to only use the GPU when the computer is not in use. This is the default setup.
If you do not want to use the GPU at all, that's also possible. This is set through the project preferences, or you can just disable the GPU completely through the client configuration file.