The use case described can’t be done using Gravity Forms out of the box. You would need to hire someone to create a custom solution for you.
That said, if you need to use some JS for your form, you’ll want to use the gform_post_render filter to ensure your code runs always when the form is available.
You can also enable ajax submission for the form to prevent page reload. If you’re using the shortcode just add ajax=“true” to the shortcode parameters.