My Wp install had some interesting problems after the 1.3.8 upgrade where (if you were logged in) the /wp-admin/ URI presented a white / blank page.
The solution that worked for me is as follows:
Download the last known working version of the plugin from the Gravity Help site's Download page (http://www.gravityhelp.com/downloads/)
Then FTP into your site and delete the gravityforms folder in your wp-content/plugins folder.
Go to your admin and see if it loads.
Then reupload the plugin you downloaded via FTP and try activating it again.
Thanks to Carl for helping me out on New Years Eve!