Adding a two-factor authentication website is pretty simple.
Just follow the steps below:
First, try whitelisting the Content Gorilla's server IP and see if then the app is able to access your site.
To do that, go to WordFence > Login Security
And add the following IP address
68.183.211.233
As shown here:
After doing this, try adding your site into Content Gorilla and see if that works.