ericjuden

This plugin allows you to use $_COOKIE, $_SERVER, $_SESSION, $_GET, $_POST and $_REQUEST data in a Gravity Form through the merge tags of a field. I’ve only really tested this using a hidden field.

Gravity Forms Extended Merge Tags

It's amazing how few people seem to know about this plugin. Incredibly simple, but also really helpful when using $_SESSION variables to track website visitors.
I use it to get referring website data, so I can easily store it in a $__SESSION variable. Then when the visitor fills out a Gravity Form, I can pass along the stored vars.
I crafted a Lead Source Tracking for Salesforce script that uses this plugin to pass along the details to Salesforce when a form is submitted.



Brilliantly simple

This plugin allows you to use $_COOKIE, $_SERVER, $_SESSION, $_GET, $_POST and $_REQUEST data in a Gravity Form through the merge tags of a field.

stefanocanziani

Add handling of user session inside SQL Database. This session manager is useful if you have an environment with more than one frontend server and a load balancer who switch the web traffic dinamically between frontend hosts. 
Mysqli extension required.

WP Database Session Handler

Add handling of user session inside SQL Database. This session manager is useful if you have an environment with more than one frontend server and a l …

Gravity Forms Extended Merge Tags vs WP Database Session Handler