How to pass post title (which could include &) to Gravity Form

You already said you tried urlencode(), but what did you try that with? If you’ve only tried urlencode() with values from the_title() or the_title_attibute() so far, try it with get_the_title() directly. I’m able to get different results by trying this out if I try with a little test shortcode:

1. Using the_title():

   function wpse_123927_cb() {
       return urlencode( the_title() );
   }
   add_shortcode('test_wpse_123927', 'wpse_123927_cb');
   

2. Using the_title_attribute():

   function wpse_123927_cb() {
       return urlencode( the_title_attribute() );
   }
   add_shortcode('test_wpse_123927', 'wpse_123927_cb');
   

3. Using get_the_title():

   function wpse_123927_cb() {
       return urlencode( get_the_title() );
   }
   add_shortcode('test_wpse_123927', 'wpse_123927_cb');
   

The third solution here works well both ways. I used this little code here to test:

function wpse_123927_cb() {
    $encode = esc_attr( urlencode( get_the_title() ) );
    $decode = urldecode($encode);

    return 'Encoded: ' . $encode . '<br>Decoded: '. $decode;
}
add_shortcode('test_wpse_123927', 'wpse_123927_cb');

And got the following output:

Hope that helps!