Gel companies that entered the market at a later date simply adopted the 100-calorie portion size. Sure, just check out the pharmaceutical industry. Several things influence the life of a gel. This lengthens its shelf life. However, flavor does change with time.
Plastic is porous.
Flavor will change, and the effectiveness of the ingredients will diminish. If you don't use it in a few weeks, throw it out. It's best to play it safe. As with any food, excessive temperatures speed up chemical reactions, which degrade the flavor. GU sitting in the trunk of a car in the summer sun for weeks on end may not taste quite as great as normal.