A "cycled" tank is a tank that has been set up with water and has a good bacteria growth in it. If you got a "used" tank from your brother, does it have water in it now? I was confused at your question about what to put in the tank. You made it sound like it was empty.
Generally, if you only have one female guppy in this tank, and if you have some hiding places like floating live plants or bushy plastic ones, then the fry will be OK. Keep the female well fed. One fish most likely will not eat all of the fry. I have a wild strain of guppies, they do not eat their fry at all. I have several adults and fry living happily together.
Live plants are better. The fry will eat the microorganisms that live around the live plants. If you try live plants, make sure you have enough light. For the 5 gallon, get the flourescent strip light that fits this tank. That should be adequate light. Leave the light on for 10-12 hours per day. Put it on a timer.
About the gravel. Some hobbyists use it others don't. I have gravel in my guppy tank. The thing is, guppies breed a lot. If you lose a couple fry, don't worry, you will have many, many more. You will need gravel if you have rooted live plants.
--Tim
