Hopefully, this helps those who are still trying to get this to work. I found this file:
/wp-content/plugins/gravityforms/includes/setup-wizard/endpoints/class-gf-setup-wizard-endpoint-validate-license.php
and changed this:
line 49: $is_valid = $info->can_be_used();
to
line 49: $is_valid = true;
Then put anything in the license key area and it will be valid.
