example.php

Let's start with a simple tag :
<gist>4545777</gist>

Or with a Gist url : 
<gist>https://gist.github.com/4545777</gist>

And with its .js extension
<gist>https://gist.github.com/4545777.js</gist>

Even better : for a specific file of your Gist
<gist>https://gist.github.com/4545777.js?file=example.php</gist>

Finally : with the entire <script /> code
<gist><script src="https://gist.github.com/4545777.js"></script></gist>

You will find easily a solution for the integration of your own Gist into mediawiki

Gist.php

<?php
# GitHubGist MediaWiki extension v.1
#    Author: Danezis Kevin 
# 		based on the work of Shohei Yokoyama (Shizuoka Univ. Japan)
# 
#-Install
# 1. Save this code as $MediaWikiRoot/extensions/Gist/Gist.php
# 2. Append "require_once "$IP/extensions/Gist/Gist.php";" to LocalSetting.php
#
#-Usage
# Let's start with a simple tag :
# 	<gist>4545777</gist>
# 
# Or with a Gist url : 
# 	<gist>https://gist.github.com/4545777</gist>
# 
# And with its .js extension
# 	<gist>https://gist.github.com/4545777.js</gist>
# 
# Even better : for a specific file of your Gist
# 	<gist>https://gist.github.com/4545777.js?file=example.php</gist>
# 
# Finally : with the entire <script /> code
# 	<gist><script src="https://gist.github.com/4545777.js"></script></gist>
# 
# You will find easily a solution for the integration of your own Gist into mediawiki

$wgExtensionFunctions[] = 'wfGist';
$wgExtensionCredits['parserhook'][] = array(
        'name' => 'Gist',
        'description' => 'Integrating your Gist code.',
        'author' => 'Danezis Kevin',
        'url' => 'http://www.furie.be/'
);

$loadMeOnce = false;
 
function wfGist() {
        global $wgParser;
        $wgParser->setHook('gist', 'wfGist_func', null);
}
 
function wfGist_func($input,$arg, $parser) {	
		global $loadMeOnce;	
		
		$input = trim($input);
		
		$sid = null; 
		$script = null;
		
		// <script src="https://gist.github.com/xxx.js"></script>
		if(preg_match("/^<script/",$input)){
			try{
				$script = @new SimpleXMLElement($input);
			} catch (Exception $e) {
				return "<b>ERROR: Gist param ( ".htmlentities($input)." ) is not a valid url.</b><br/>";
			}
			$input = $script["src"];
		}
		
		
		if (preg_match("/^[0-9a-f]+ ?/",$input)){
            $sid = $input;
			$script .= '<script src="https://gist.github.com/'.$sid.'.js"></script>';
        }elseif (preg_match("/^https\:\/\/gist\.github\.com\/(?P<sid>[0-9a-f]+)$/",$input, $matches)){
			$script = '<script src="'.$input.'.js"></script>';
			$sid = $matches['sid'];
        }elseif (preg_match("/^https\:\/\/gist\.github\.com\/(?P<sid>[0-9a-f]+)\.js(\?file\=.*)?$/",$input, $matches)){
			$script = '<script src="'.$input.'"></script>';
			$sid = $matches['sid'];
        } else {
			return "<b>ERROR: Gist param ( ".htmlentities($input)." ) is not a valid id.</b><br/>";
		}
		
		
	//	echo "sid = ".$sid." script = ".htmlentities($script)."</br>";
		
		$html = '';
		
		if(!$loadMeOnce) {
			$html .= <<<'HTML'
	<style>
		.gistWrapper{
			margin-bottom: 5px;
		}
		.gist .gist-file .gist-data .line_numbers span {line-height: 1.3em;}	
		span.info{
			background-color: #F3F3F3;
			border-bottom: 1px solid #7E7E7E;
			border-top: 1px solid #7E7E7E;
			display: inline-block;
		}
		span.first{
			border-left: 1px solid #7E7E7E;
			border-radius: 12px 0 0 12px;
			
		}
		span.last{
			border-right: 1px solid #7E7E7E;
			border-radius: 0 12px 12px 0;
		}
		span.info:hover{
			text-decoration: underline;
			background-color:#79C995;
		}
		span.info a{
			color: #000000;
			display: inline-block;
			height: 27px;
			line-height: 2.1em;
			padding: 0 10px;
		}
		span.gist_logo{
			background-image: url(data:image/png;base64,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);
			background-repeat: no-repeat;
			padding-left: 62px;
		}
		.blocGist{
			display:none;
			margin-top: 10px;
		}
		.clear{
			clear:both;
		}
	</style>
	<script src="http://ajax.googleapis.com/ajax/libs/jquery/1.9.0/jquery.min.js"> </script>
	<script type="text/javascript">$("document").ready(function() {$(".hide").click(function () { $(this).next("div").toggle("slow"); return false; });});</script>			
HTML;
			}		
			
			$html .= <<<HTML
			<div class='gistWrapper'>
				<span class='info first gist_logo'>
					<a target='_blank' href='https://gist.github.com/$sid/edit'>&Eacute;diter</a>
				</span>
				<span class='info'>
					<a target='_blank' href='https://gist.github.com/$sid#comments'>Commentaires</a>
				</span>
				<span class='info'>
					<a target='_blank' href='https://gist.github.com/$sid/revisions'>R&eacute;visions</a>
				</span>
				<span class='info hide last'>
					<a href='#'>D&eacute;rouler</a>
				</span>
			<div class='blocGist'>$script</div>
			</div>
HTML;

		
		
		$replace = array(CHR(13),CHR(10),CHR(9));
		$html = str_replace($replace,"",$html); 
		
		$loadMeOnce = true;
		return $html;
}

Correction : gist's url allow Hexa code : 0-9 and a-f

I rewrote the code for my own needs.