Adding support for editor font sizes

gutenberg-font-sizes.php

<?php

function prefix_setup_theme() {
	
	add_theme_support( 'editor-font-sizes', array(
	    array(
	        'name' => __( 'Small', 'themeLangDomain' ),
	        'size' => 12,
	        'slug' => 'small'
	    ),
	    array(
	        'name' => __( 'Normal', 'themeLangDomain' ),
	        'size' => 16,
	        'slug' => 'normal'
	    ),
	    array(
	        'name' => __( 'Large', 'themeLangDomain' ),
	        'size' => 36,
	        'slug' => 'large'
	    ),
	    array(
	        'name' => __( 'Huge', 'themeLangDomain' ),
	        'size' => 50,
	        'slug' => 'huge'
	    )
	) );

}

add_action( 'after_setup_theme', 'prefix_setup_theme', 10 );