Netscaler / Access Gateway 401 - Unauthorized: Access is denied due to invalid credentials

Setting up the Access Gateway on a Netscaler to replace an 2010 appliance.
Found this guide to be very good for initial setup:
http://blogs.citrix.com/2012/04/10/netscaler-for-the-xendesktopxenapp-dummy/

After setup though we where still having issues with the following error:
401 - Unauthorized: Access is denied due to invalid credentials
You do not have permission to view this directory or page using the credentials that you supplied.

Looking at the Web Interface logs we found:
Event ID:      18001
A communication error occurred while attempting to contact the Access Gateway authentication service at https://xxxxxxxx/CitrixAuthService/AuthService.asmx. Check that the authentication service is running. The message reported by the underlying platform was: Unable to connect to the remote server. [Unique Log ID: 88ee87ee]

With this I was able to find Shaun Ritchie's excellent blog:
http://www.shaunritchie.co.uk/access-gateway-401-unauthorized-access-is-denied-due-to-invalid-credentials

Resolution for us was to add an entry into the hosts file pointing the FQDN to the Access Gateway virtual server internal IP address.
Note: can also be changed in DNS for your domain.