Make sure that the Elements module in GeneratePress is enabled (Appearance » GeneratePress » Elements).
Then head to Appearance » Elements » Add New Element.
Choose Block and click Create.
In the Element settings on the right, choose Hook as your Element Type and select Custom Hook as Hook Name. Then, in the Custom Hook Name field, enter:
generate_do_template_part
Give the Element a meaningful title, such as Custom 404, and set the Location to 404 Template.
Now, you can add any Gutenberg blocks to style your custom 404 page and hit Publish once you’re done.
I like to add a search bar and the latest blog posts to help visitors find the content they’re looking for, but you could also add some kind of call to action, such as a freebie or email opt-in form.
Feel free to get creative. 🙂
This is what I chose for the Kind Blogger website: