It’s not just problem of Thrive and GeneratePress.
Any page builder tend to have problem with fullwidth layout if it’s not theme built-in.
But especially Thrive Builder has no option to adapt to fullwith if it’s not with Thrive’s theme.
But at least to have page builders adapt to themes, themes should have fullwidth layout template.
Which seems to be not in GeneratePress now.
Would you add “only content is fullwidth” layout template?