Answer:
GENERAL LIGHTING SERVICE (LAMP)
global light source
Used to refer to what is more commonly called an "incandescent bulb"
GLS stands for General Lighting Service (lamp/bulb). They are the standard light bulbs you are probably used to seeing around your home.
There is a pretty decent descritpion here..
http://www.lampshoponline.com/categories/Lamps/General-Light-Bulbs/Normal-GLS-Light-Bulbs/