GMC 2003 C4C042 Owner's Manual page 270

C6/C7/C8 Models
CAUTION:
Fuel vapor is highly flammable. It burns
violently, and that can cause very bad injuries.
Don't smoke if you're near fuel or refueling your
vehicle. Keep sparks, flames and smoking
materials away from fuel.
6-16
The fuel cap can be on either or both sides of your
vehicle depending on option content.
To take off the cap, turn it slowly to the left
(counterclockwise).
CAUTION:
Gasoline Engine Vehicles: If you get gasoline on
you and then something ignites it, you could be
badly burned. Gasoline can spray out on you if
you open the fuel cap too quickly. This spray can
happen if your tank is nearly full, and is more
likely in hot weather. Open the fuel filler cap
slowly and wait for any "hiss" noise to stop. Then
unscrew the cap all the way.
When you put the cap back on, turn it to the right
(clockwise) until it is tight.