Downgrading isn't the solution. Upgrading Gravity Forms is.
WordPress v3.6 introduced a new version of jQuery. Gravity Forms, like all plugins and themes, utilized the version of jQuery that is built into the WordPress core.
This change necessitated updates to Gravity Forms to maintain compatibility. So in order to run WordPress v3.6 and Gravity Forms you must have the latest version of Gravity Forms.
This is why renewing Gravity Forms in order to continue receiving software updates is important. You should ALWAYS run the latest version of WordPress AND the latest version of all WordPress plugins for security and compatibility reasons.
Downgrading the version of WordPress you use in this situation could leave your site vulnerable to any security fixes applied in the WordPress v3.6+ release. Same goes for running old versions of Gravity Forms.