WP_User_Query Filter Results

Mostly a PHP question but if I understand your question, you want something like this:

$args  = array(
'role'      => 'talent',
'order'     => 'DESC',
'orderby'   => 'user_registered',
'meta_key'  => 'position',
'number'    => '15'
);
$meta = array (
    array(
        'key'       => 'picture',
        'value'   => array(''),
        'compare' => 'NOT IN'
    ),
    array(
        'key'       => 'position',
        'value'     => $title
    )
);

$orderby_options = array(
  'All' => 'All',
  'Male' => 'Male',
  'Female' => 'Female',
);
//  var_dump($_GET);
if (!empty($_GET['gender']) && in_array($_GET['gender'],$orderby_options)) {

 $meta[] = array(
   'key' => 'gender',
   'value' => $orderby_options[$_GET['gender']]
 );
}