Do you want to disable them for all post types? If so:
in the Gutenberg editor > Show More Tools & Options ( 3 dot ) menu in the far right of topbar select Options where you can disable any meta boxes you dont want. This setting will be saved as the default for the user.
Note that in this code, we’re referencing side as that’s where the metabox is in GP 2.3, which is set to be released next week. You can install the beta now for this code to work: https://generatepress.com/generatepress-2-3/