GMC 2000 Sonoma Owner's Manual page 250

To remove the cap, turn it slowly to the left
(counterclockwise). The cap has a spring in it; if you let
go of the cap too soon, it will spring back to the right.
6-8
CAUTION:
If you get fuel on yourself and then something
ignites it, you could be badly burned. Fuel can
spray out on you if you open the fuel filler 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.
Be careful not to spill fuel. Clean fuel from painted
surfaces as soon as possible. See "Cleaning the Outside
of Your Vehicle" in the Index.