Assuming you need to siphon the gas out of your 2005 Chevy Silverado for some reason, there are a few ways to do it.
The easiest way is to use a garden hose. First, make sure the hose is clean and free of any debris. Next, insert one end of the hose into the gas tank and secure it with tape or a rubber band. Then, suck on the other end of the hose until you get a mouthful of gasoline (this may take a few tries). Finally, put the end of the hose in whatever container you're using to store the gas and let gravity do its thing. Once the flow of gas slows to a trickle, remove the hose and seal up your container.
If you don't have a garden hose handy, you can use a length of tubing instead. Again, make sure it's clean before proceeding. Insert one end of the tubing into the gas tank and secure it with tape or a rubber band. Put your mouth over the other end of the tubing and blow gently until you see gasoline flowing through (this may take a few tries). Quickly put that end of the tubing into your container so as not to lose any gas, then remove when finished and seal up your container.
There are also commercial products available designed specifically for siphoning gasoline - these usually consist of a pump that attaches directly to your car's gas tank opening.