Although not in the documentation, 'firstsortorder' => 'desc' does the trick.
$grid->setGridOptions(array(
'firstsortorder' => 'desc'
"caption"=>"Player Statistics",
"rowNum"=>200,
"rowList"=>array(50,200,1000),
"sortname"=>"percentOwned",
"sortorder" => 'desc',
"width" => 1000,
"height" => 400,
"hoverrows" => true,
"viewrecords" => false
));