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']]
);
}