functions.php

<?php
wp_embed_register_handler( 'gist', '/https?:\/\/gist\.github\.com\/([a-z0-9]+)\/([a-z0-9]+)(#file=.*)?/i', function( $matches, $attr, $url, $rawattr ) {
	$embed = sprintf(
		'<script src="https://gist.github.com/%1$s/%2$s.js"></script>',
		esc_attr( $matches[1] ),
		esc_attr( $matches[2] )
		);

	return apply_filters( 'embed_gist', $embed, $matches, $attr, $url, $rawattr );
} );
?>