The GLO whitening system is a pretty cool product, and I have to admire the elegance of the the system. However, if you take a look at the ingredients in the G-Vials you can see both why it will make your teeth whiter and why it isn't a healthy or sustainable option for whitening your teeth.
Here is what is in the G-Vials:
Water/Eau (Purifed/ Purifiée), PVP-Hydrogen Peroxide, Glycerin, VP/VA Copolymer, Poloxamer 407, Carbomer, Flavor (Aroma), Etidronic Acid, Ammonium Hydroxide, Sucralose, Sodium Stannate.
That list of ingredients probably doesn't mean a whole lot to most people, but what jumps out at me is the Etidronic Acid.
Etidronic acid is a powerful calcium chelating agent, which means it will pull calcium out of your tooth enamel. Ironically, the substance also inhibits radical formation, which is actually what you want the hydrogen peroxide to do in order to break up stains. The bottom line is that it will get your teeth white, but it will do it by stripping layers off the outside of your teeth and thus have unhealthy side effects on your enamel.
Crest White Strips (though not well liked by most dentists) is a much healthier alternative for at-home whitening if done infrequently. If you are interested in the best results with limited negative effects on your enamel you should get whitening done through your dentist.