When my tank is full and i'm start the engine; the gauge display full but after 50 secondes; the needle falling to empty. Does the fuel level sensor is accessible by the interior or you must remove the fuel tank ?
What year is your car? If it's new, take it in for warranty. If it's pre-2002/3 (I believe) GM had issues with their fuel sensors. The sulfur in the gas coroded (or something like that) the sensor and caused issues with accuracy. If that is the case, you'll have to drop the tank and replace the fuel sensor. If you have a newer vehicle, I honestly don't know what to say. It could be the same issue, a problem with the gauge, or something else. If you have a 2004 or older you may want to hit up www.grandamgt.com/forum and search around there. I'm sure there are probably instructions on how to replace it yourself. But, if not, there's a wealth of knowledge on that board that should be able to help you.