Keeping CSS model open after form submission

A combination of PHP and JS will do it.

1. Have your page listen for the presence of gform_submit which Gravity Forms includes as a hidden field on all forms by default.

   if(!empty($_POST['gform_submit'])) {
       // now here comes your javascript
   }
   

2. For the JS, set modal-state checked. This way on page load, if the form has been submitted, the JS will trigger the CSS which makes the modal visible.

   <script type="text/javascript">
       jQuery(document).ready(function(){
           jQuery("#modal-1").prop("checked", true);
       });
   </script>
   

You can do it in vanilla JS as well, but I’m assuming if you’re using modals and the like you probably have jQuery available.

Not sure what your use case is, but it would be wise to check the accessibility of your solution to screen readers, keyboard users, etc.