Support for Post Thumbnails & Custom Thumbnail Sizes in WordPress and Add those sizes to Block Editor/Gutenberg

wp-image-sizes.php

<?php
add_action( 'init', function() {
add_image_size( 'rename-this', 1200, 500, false );
add_image_size( 'rename-again', 550, 550, true );
} );

// Add these sizes to Gutenberg
add_filter( 'image_size_names_choose','md_custom_image_sizes_gb' );

function md_custom_image_sizes_gb( $sizes ) {
	return array_merge( $sizes, array(
	//Add your custom sizes here
	'rename-this' => __( 'Banner' ),
	'rename-again' => __( 'Block' )
	)
					 );
}
?>

‘true’ and ‘false’ are answers of argument, whether to force-crop the image or not.