Hi there,
i would do this:
1. Make the 404 Template full width with a Layout Element in Appearance > Elements:
https://docs.generatepress.com/article/layout-element-overview/
Set the Content containe to Full Width.
Set the Display Rules > Location to: 404 Template
2. Then create a Block Element:
https://docs.generatepress.com/article/block-element-overview/
Set the Element type to: Content Template
Set the Display Rules > Location to: 404 Template
Then in the editor construct your 404 page. I would recommend using GenerateBlocks to construct your page.