William P. Davis

This plugin allows you to force certain pages to display as SSL-only by simply checking a box. Please note, you must have SSL configured on your server and require SSL for either login or admin.

Define SSL Pages

Allows you to easily require SSL for specific pages.

Timothy Wood

An addon to Gravity Forms to add an option to force your forms to be loaded SSL only. Fully supporting the latest version of Gravity Forms 1.9!
This plugin requires Gravity Forms 1.7+ and is tested through WordPress 4.1.
Thanks to the following users for making the plugin better! 
* @limecanvas
Additional Details:
<ul>
<li>This plugin is actively supported and we will do our best to help you.</li>
<li>This plugin has been tested on PHP 5.2.17, 5.3.14, 5.4.4 and WP 3.4, 4.0, and 4.1.</li>
<li>Background patterns used in the WordPress.org banner were created by http://subtlepatterns.com</li>
</ul>

Gravity Forms: Force SSL

An addon to Gravity Forms to add an option to force your forms to be loaded SSL only.

Define SSL Pages vs Gravity Forms: Force SSL