In order to turn a typical sunset into an extraordinary sunset, you are going to do the opposite of counteracting your available light. You do this by picking the colored gel that is the opposite color of the color you want to highlight. Though it may seem like an odd idea, it's actually just simple color theory. The opposite color of magenta is green. By placing a light to medium green gel on your strobe and setting your camera's white balance (WB) to fluorescent, anything that is magenta (such as a sunset) will be pushed even more vibrant.
Some strobes, such as Nikon speedlites, come with an orange and a green gel. These are the most commonly used gels because they are roughly color-balanced to fluorescent and tungsten light, which is how most rooms are lit. Normally when you use gels, you are counteracting the environmental light. For example, if you are in a tungsten-lit room and added a strobe light, the color difference between the room light and the strobes will be very apparent. By adding an orange gel to your speedlite and setting your WB to tungsten, all the light in the room will now be uniform and natural looking.
I'd love to see your experiments with this technique. And it doesn't necessarily need to be this green-to-magenta scenario. Get creative. Make sure to drop links to your photos in the comments and I will post my favorites in the next lighting post. Happy shooting!
Lessons like this one as well as 25 other lighting diagrams are available in my new e-book, RGLR, The Run & Gun Lighting Resource for $10.