Thanks for this, I added this code:
add_filter( 'generate_typography_customize_list', 'tu_add_google_fonts' );
function tu_add_google_fonts( $fonts ) {
$fonts[ 'zilla_slab' ] = array(
'name' => 'Brawler',
'variants' => array( '300', '300i', '400', '400i', '500', '500i', '600', '600i', '700', '700i' ),
'category' => 'serif'
);
return $fonts;
}
And although it says the headings are Brawler here, it does not seem to have updated the fonts of headings?