Just wanted to share my solution to this topic http://www.gravityhelp.com/forums/topic/customizing-activation-pageoutput?replies=8#post-102243
This code goes into your theme's functions.php file.
Changing the subject was the straightforward part:
To change the message I essentially combined the gf user registration filter with the core code so I could change the message without disrupting the process:
You can change that text to anything you want instead of the vanilla wp text/email.