The solution for this problem is simple. What you have to do is to exclude specific css and js files from being cached by the caching plugin. In the case of Gravity forms, these files are :
wp-content/plugins/gravityforms/css/formreset.css wp-content/plugins/gravityforms/css/datepicker.css wp-content/plugins/gravityforms/css/formsmain.css wp-content/plugins/gravityforms/css/browsers.css
Those files mustn’t be minified by the caching plugin you use.
Another approach is to exclude the pages which uses GravityForms, but that will be, in my opinion, not efficient. Because you don’t have to update the exclusion rules every time you add a new page with a GravityForm.
After applying the mentioned steps, the captcha must be visible.
I hope this helped you 🙂