Hello,
I'm having the same problem as described here:
http://www.gravityhelp.com/forums/topic/problem-with-form-communicating-with-authorizenet
Because the issue was resolved via email in that post, I don't know what the resolution was. Our form goes through and shows the captured info on the admin side, but there is no corresponding transaction on our Authorize.net, not even under unsettled transactions. What could be the problem?