GMC C series 2002 Owner's Manual page 211

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.
6-8
When you put the cap back on, turn it to the right
(clockwise) until it is tight.
NOTICE:
If you need a new fuel cap, be sure to get the
right type. Your dealer can get one for you.
If you get the wrong type, it may not fit or have
proper venting, and your fuel tank and emissions
system might be damaged.
If you have dual tanks, fill the driver's side first. The
fuel gage will give better readings this way. Your gage
will show the average level of both tanks combined.