I think we might be having the same issue as discussed here:
http://www.gravityhelp.com/forums/topic/user-registration-no-longer-working
We're on the most up-to-date version of GF and GF User Registration. The form is working on the desktop version of our site, but on the mobile version (calling the same exact form) the form won't submit. When I click "submit" the page just reloads.