Because gels are thin and not very stable, they all eventually fade and sometimes burn. Light fixtures on long burning schedules need to be regelled often. One solution is to sandwich the gel between two plates of glass. This reduces the amount of heat and UV exposure, adding life to the gel. The great thing about gels is that they're relatively inexpensive and they're readily available.
Gels are also flexible and can be formed into sleeves for fluorescent tubes.
A more durable, permanent type of filter is made of glass, composed of either molded glass or glass plates. Glass filters work in the same way as gels but are much more stable, easily handling much higher temperatures and long-term exposure. Newer glass filters are made of strips of colored glass held in metal frames. Because most lamps do not have an even distribution pattern, differential heat expansion causes some lenses to break. The glass strips may shift microscopically but rarely break.