Gravity Forms is an extremely popular contact form plugin for WordPress. One of the reasons Gravity Forms is so popular is the ease in which you can customize it. Much like WordPress, Gravity Forms features various action and filter hooks for developers to use to easily tweak how GF functions.
As you can see I’m using the gform_validation_message filter to customize how the error message is processed. The PHP variable $validation_message stores the original error message GF was going to display. In this example I didn’t use the original error message, but you could easily pass that to your JS popup if needed.
This is a pretty simple example of customizing the Gravity Forms error message. Enjoy!