A gel is an apparently solid, jellylike material formed from a colloidal solution. By weight, gels are mostly liquid, yet they behave like solids. An example is gelatin.

In fiber-optic communications, a gel resembling petroleum jelly in viscosity is used to surround a fiber, or multiple fibers, enclosed in a loose buffer tube. This gel serves to lubricate and support the fibers in the buffer tube. It also prevents water intrusion if the buffer tube is breached. Gels are also used in fiber-optics as index-matching materials.

Source: from Federal Standard 1037C and from the FAA Glossary of Optical Communications Terms

In optical filters and theatrical lighting a color gel is a transparent or translucent colored panel used to change the color of transmitted light.