GeneratePress sets the site header as H1 when the front page is set as the blog page.
You may modify this behavior through a filter. Can you try adding this PHP snippet:
add_filter( 'generate_site_title_output', function( $output ) {
return sprintf(
'<%1$s class="main-title"%4$s>
<a href="%2$s" rel="home">
%3$s
</a>
</%1$s>',
'p',
esc_url( apply_filters( 'generate_site_title_href', home_url( '/' ) ) ),
get_bloginfo( 'name' ),
'microdata' === generate_get_schema_type() ? ' itemprop="headline"' : ''
);
});
Adding PHP reference: https://docs.generatepress.com/article/adding-php/#code-snippets
Kindly let us know how it goes.