How to add external remote JS source to Gravity Forms?

I think you’re looking for gform_enqueue_scripts which is simply a Gravity Forms specific version of the wp_enqueue_scripts hook. Either hook uses the wp_enqueue_script() function.

That function can take a local script—usually using get_template_directory_uri() (theme), get_stylesheet_directory_uri() child theme), or plugins_url() (plugin) to reference the file location—or an external location like what you’re trying to do. You’d probably load both the external JS and your custom file on that same hook with two calls to wp_enqueue_script() with your custom file listing the external file as a dependency in the $deps arg.

What goes in that custom sheet is out of scope of this forum as it’s plugin specific, but this should get you going.