Here’s how to add PHP – https://docs.generatepress.com/article/adding-php/
You can pick between adding a child theme or using the Code Snippet plugin.
Code Snippet plugin may be easier as it doesn’t require you to create child theme files for upload.
Within this code:
add_filter( 'generate_logo_output', 'tu_logo_class', 10, 3 );
function tu_logo_class( $output, $logo_url, $html_attr ) {
printf(
'<div class="site-logo MY-CUSTOM-CLASS">
<a href="%1$s" title="%2$s" rel="home">
<img %3$s />
</a>
</div>',
esc_url( apply_filters( 'generate_logo_href' , home_url( '/' ) ) ),
esc_attr( apply_filters( 'generate_logo_title', get_bloginfo( 'name', 'display' ) ) ),
$html_attr
);
}
Replace “MY-CUSTOM-CLASS” with your CSS class.