I reviewed the forums and found reference to a problem that I'm also having. The topic can be found here: http://www.gravityhelp.com/forums/topic/16411-bug-existing-custom-fields-not-being-recognized
I read the reply indicating that there needed to be posts created with the post meta filled out before GF would recognize the filed as "existing" in the existing field name drop down.
I'm using GF with Testimonials by WooThemes to create a user submission forum for testimonials. I have been able to successfully map GF to the Testimonials custom post type but not it's fields.
I did create/publish several test testimonials with all the Testimonials fields filled out but GF still doesn't not recognize them as existing. Testimonials uses the following fields:
gravatar_email, byline, url
These fields are all stored in an array so I'm not sure if that might be the problem in why they are not showing up? Can anyone point me in the right direction?