Integrate the Custom Fonts (https://wordpress.org/plugins/custom-fonts/) plugin with the GP Customizer.

gistfile1.txt

add_filter( 'generate_typography_default_fonts', function( $fonts ) {
	if ( ! class_exists( 'Bsf_Custom_Fonts_Taxonomy' ) ) {
		return $fonts;
	}

	$all_fonts = Bsf_Custom_Fonts_Taxonomy::get_fonts();

	if ( ! empty( $all_fonts ) ) {
		foreach ( $all_fonts as $font_family_name => $fonts_url ) {
			$fonts[] = $font_family_name;
		}
	}

	return $fonts;
} );