That is a *fantastic* question. There is no such thing, it's a nickname.
GTLS. Gaseous Tritium Light Source is the more complete and correct name. These are borosilicate glass vials, tubes, rectangles, spheres, dots and more which are internally coated with a dyed phosphor. Then they are filled with Tritium (Hydrogen 3) gas. This is radioactive and the beta particles expended from the decay of the tritium excite the phosphors which glow their dyed color.
Folks talk about the radioactive half life of Tritium (about 12.5yrs) but it seems that the quality of the phosphor/dye and the handling (shock, light exposure) of the object may be more contributory to light output over time. I've got EDC items only 6yrs old that are somewhat dimmer than you might expect for instance. And other items I've seen (15yr old watch mostly kept in a drawer) still glowing nicely.
There was a big rush to stuff as many GTLS vials as possible into flashlights for a while, and while they are lovely, they are not exactly timeless
I've found that a light with no vials is harder to find in the dark. Not just if it's 'lost' but on a dark table, it's so easy to pick out with some luminous devices on it's skin. So I like 4-8 on a light these days. YMMV of course.