Mmk, would this work?

$gt = str_replace(" ","-",$_GET['gt']);

EDIT:
I also forgot to mention I'm using mod_rewrite and that the actual URL is
/stats/stats.php?gt=Gamertaghere

@sketchMedia
I posted working links in my post.

EDIT2:
Thanks delayedinsanity, str_replace worked perfect. I ended up using this:
$gt = str_replace(" ","+",$_GET['gt']);