Here are a couple items from the GD help / knowledge base:
NOTE: The automatic install described below will not work on free hosting accounts. WordPress needs to be manually installed on free hosting accounts, however, certain WordPress features may not work correctly in the ad-supported environment.
NOTE: If your website contains certain advanced features, such as ASP, ASP.NET, CGI, or PHP applications and you select a hosting plan that does not support those advanced features, your website may no longer function properly after the plan change. Please make sure your website does not contain advanced features BEFORE moving to a plan that may not support them or be prepared to modify your Web content accordingly.
If all you want to do is testing/debugging and already have a paid account, then just install WP in a sub directory :) Heck, I've got at least a dozen WP, Joomla, Drupal, eFront installs running on a GD shared Linux plan :)