gravityflow_post_update_post

add_action( 'gravityflow_post_update_post', 'jo_post_update_featured_alt', 10, 3 );
function jo_post_update_featured_alt( $post, $feed, $step ) {
    //Adjust for which step(s) you want to update alt tags with
    if ( ! $step->get_ID() == '33' ) {
        return;
    }

    $post_id      = $post['ID'];
    $thumbnail_id = get_post_thumbnail_id( $post_id );
    $entry        = $step->get_entry();

    if ( $thumbnail_id ) {

        //Identify based on feed ID which entry field contains the alt text.
        switch ( $feed['id'] ) :
            case '1037':
                $alt_text = $entry['4'];
                break;
            case '1038':
                $alt_text = $entry['150'];
                break;
            case '78':
                $alt_text = $entry['4'];
                break;
            default:
                $alt_text = false;
        endswitch;

        if ( $alt_text ) {
            //Update the _wp_attachment_image_alt meta for the post with the new text.
            $result = update_post_meta( $thumbnail_id, '_wp_attachment_image_alt', $alt_text );
        }
    }
}