OK try removing everything you have before and just add this:
add_filter( 'generate_post_author_output', 'tu_add_author_gravatar' );
function tu_add_author_gravatar() {
printf( ' <span class="byline">%1$s</span>',
sprintf( '<span class="author vcard" itemtype="http://schema.org/Person" itemscope="itemscope" itemprop="author">%1$s %5$s<span class="author-name" itemprop="name">%4$s</span></span>',
__( 'by','generatepress'),
esc_url( get_author_posts_url( get_the_author_meta( 'ID' ) ) ),
esc_attr( sprintf( __( 'View all posts by %s', 'generatepress' ), get_the_author() ) ),
esc_html( get_the_author() ),
get_avatar( get_the_author_meta( 'ID' ) )
)
);
}
add_filter( 'generate_post_date_output','tu_remove_date_link', 10, 2 );
function tu_remove_date_link( $output, $time_string ) {
printf( '<span class="posted-on">%s</span>',
$time_string
);
}