Ubuntu – Why would an .htaccess file result in 403 Forbidden when the only line is “Allow from all”

not a straight answer, but if you have php [ or other server side include scripting ] installed - you can use:

ErrorDocument 403 /404.php

and in 404.php have:

<?php
  header("HTTP/1.0 404 Not Found");
  //...

You don't need that RewriteBase in there, try this:

RewriteEngine on
RewriteRule ^/gateway\.php$ - [L]
RewriteCond %{REMOTE_HOST} ^127\.0\.0\.1
RewriteRule ^(.*)$ - [L]
RewriteRule ^(.*)$ http://www.website.com/gateway.php?req=%{REQUEST_URI} [R=302,L]

First you explicitly say that if gateway.php is requested, stop. Then you say if it's from localhost just do the URL, ELSE do the redirect. You may be subject to some sort of internal redirect mechanism, you might have to set RewriteLog and RewriteLogLevel up right quick to debug the actions Apache is taking.