A GravityForms form rendered with Popup Maker will not properly submit when the form includes a CAPTCHA field.  GravityForms’ own documentation states: 

“Please Note: Due to limitations of reCAPTCHA, if your form contains a reCAPTCHA field it will not be able to submit via AJAX.”

To resolve the problem, remove the CAPTCHA field from the form. 

There are several additional ways to combat spam when using forms on your site:

1) Select a ‘honeypot’ setting in your form. 

2) Use a spam detection and filtering plugin, like Automattic’s  Akismet or AntiSpam Bee from the WordPress.org plugin repository. 

Ninja Forms published an extensive blog post on the value of spam ‘honeypots’ and the pros and cons of adding a CAPTCHA field to your forms.