Set post thumbnail to a radio button value

<?php
// http://www.gravityhelp.com/forums/topic/display-featured-image-selection-based-on-conditional-logic#post-161066
// change the 98 here to your form ID
add_action('gform_after_submission_98', 'set_featured_image', 10, 2);
function set_featured_image($entry, $form){
    // change $entry['2'] to the field where the radio button
    // image was selected mine was field 2.  The value of that
    // radio button should be the attachment ID
    set_post_thumbnail($entry['post_id'], $entry['2'];);
}