When we hear the word “gel” most of us think of the stuff that people put in their hair. In making movies it has to do with lighting.
Have you ever noticed that sometimes in a sad scene the lighting appears in cool, dark colors? Or in an exciting, scene we see hot colors that get your heart pounding? The cinematographer of the film didn’t just say “I’m feeling sad today, lets make this scene blue”. He was trying to set the mood for the scene. These different lighting techniques 
convey the emotions the actors are feeling, the context of the setting and/or a meaning behind an overall message. Without this lighting technique we would miss the emotions of many scenes.
For example, in the film Kill Bill Volume 2 when we see what Beatrix Kiddo’s training was like. The lighting is red, it’s gets you, for lack of a better word, excited. We see her karate chopping and kicking like the female version of Bruce Lee. This is makes the audience get pumped up! Without these different lighting techniques, films wouldn’t convey the emotions as well as they should.
Reading this article has taught me that lighting not only makes the picture beautiful, but supplies emotion too.- dpc27
A link to the article is here http://www.videomaker.com/article/13544/