Paste the following code into your functions.php file to embed Gists with only their URL.

embed-gists.php

/**
 * Embed Gists with a URL
 *
 * Usage:
 * Paste a gist link into a blog post or page and it will be embedded eg:
 * https://gist.github.com/2926827
 *
 * If a gist has multiple files you can select one using a url in the following format:
 * https://gist.github.com/2926827?file=embed-gist.php
 */
 
	wp_embed_register_handler( 'gist', '/https:\/\/gist\.github\.com\/(\d+)(\?file=.*)?/i', 'bhww_embed_handler_gist' );
 
	function bhww_embed_handler_gist( $matches, $attr, $url, $rawattr ) {
 
		$embed = sprintf(
				'<script src="https://gist.github.com/%1$s.js%2$s"></script>',
				esc_attr($matches[1]),
				esc_attr($matches[2])
				);
 
		return apply_filters( 'embed_gist', $embed, $matches, $attr, $url, $rawattr );
		
	} // End bhww_embed_handler_gist