Trust me your better of with the GM, but besides that, most local auto parts stores will put a scanner on your truck these days and pull your codes in hopes of selling you parts. Try an Auto Zone for example.
Resetting your light, I am not model specific on this but to reset the codes and check engine light, most time all you need to do is disconnect the negative battery terminal post, and then insert the key and turn the key and hold it in the backwards accs position of 10+ seconds. You will feel that you can hold it turned backwards a little more than it will normally sit on its own. Also some times their may be a sequece of turn the key backwards hold for 10 seconds and turn the key as if you were starting it for 10 seconds and then turn all the way backagain for 10 seconds. I would advise a web search on the topic you'd be surprised by what you can find. Id search "duramax check engine code reset 2002" or something similar.
Good luck, at least its just a light, my ford blew the spark plugs outta the heads like a speedfreak spits out teeth. But I would definatley try and get the codes pulled before resetting them, the little chain stores do it for free in hopes of selling you those parts.
Jason