Hi Frederick and welcome. GLS = Glass Bedded Sir. The Browning XBLT Medallion308 GLS features include a blue steel Glass Bedded Receiver on this Right Handed Rifle. Cheers and stay safe!

Hi Frederick, your guess is as good as anyone's as to what GLS stands for. Whoever wrote the description put it in. No other vendor (or Browning) uses this. My guess is that it is short for Gloss, as in "The X-Bolt Medallion bolt action rifle is presented with a fine gloss-finished, checkered Walnut stock."
This is a right handed rifle.
Stay safe & have a great day!