User Role Editor does not make any special efforts on custom user capabilites created by other plugins. ‘gravity_forms_addonname_uninstall’ user capability is registered by active Gravity Forms add-on and should be used by Gravity Forms or/and related GF add-on.
URE helps you to add such custom user capability to a role by your selection. URE saves capability with role together in the database when you turn it on for a role. That’s it.
I made a quick search through the GF code and one of GF add-on I have. It seems, on the 1st glance, that this capability ‘gravity_forms_addonname_uninstall’ is never used.