Gravity forms is a very popular plugin for adding contact and data collection forms to your website.
It uses shortcodes for the place holders of particular forms in the post and page edit forms, which are then replaced with the relevant form on the public side of your site.
Unfortunately it appears that the Gravity forms plugin only registers (adds) a shortcode to WordPress on the public serving side of the plugin and, thus, the shortcode isn't shown in the membership plugins shortcode list.
As this problem is still in the latest version of the Gravity Forms plugin (v.184.108.40.206), rather than asking you to keep editing your Gravity Forms code, I have created an add-on plugin for the membership plugin:
You can grab it from here:
Simply unzip the plugin and place the php file inside the memerbshipincludes/plugins directory.