Set post format to user selection

<?php
// http://www.gravityhelp.com/forums/topic/set-a-post-format#post-179861
// change the number 11 here to your post ID
add_action('gform_after_submission_11', 'gf_set_post_format', 10, 2);
function gf_set_post_format($entry, $form){

    // get post ID
    $postID = $entry['post_id'];

    // get the submitted post format
    // update this number to the field where your post format is submitted
    $format = $entry['3'];

    // set post format
    set_post_format($postID, $format);
}