No, not for running a site. AtarCMS's own code is MIT. The admin's rich-text editor is the GPL build of CKEditor 5, which ships with the core as a separate, unmodified component with its own licence files.
The GPL matters only if you redistribute AtarCMS itself together with the editor: then that redistribution must honour the GPL for that component. Running your site, and the theme and content you make, are not affected.