Make a readonly field

I took a quick look and didn't see this functionality available by default but it's pretty easy to add yourself.

1. On the advanced tab for each field add a "disable" class.

   This is just a class name that we will use to reference any elements that you want disabled so all though it makes sense to call it "disable" it can actually be anything you want.

2. Make sure that you've queued jQuery and add the following to you header.php:
   

   <script type="text/javascript">
   		jQuery(document).ready(function($){
   			$(".gform_wrapper .disable input").attr('disabled','disabled');
   		});
   	</script>

   This will disable all Gravity Form fields with the disable class that are inputs. If you need to also disable a select or any other field type, you will need to specify this like so:
   $(".gform_wrapper .disable input, .gform_wrapper .disable select").attr('disabled','disabled');

Hope that helps.