I have a gmc truck with a 350 motor and the problem I am having is while driving on a flat and level road my truck has OK power but when it comes to any size hill it starts to slow down and has no power so can anyone please give me some if on what the problem could be.any info will be helpful

You need to supply a little more info like make/model etc.

Back in my high school days my father's car, a 1968 model would stall going up a hill. In one case we hod to go around. Turned out to be a popped check valve in the mechanical fuel pump.

I had a similar problem in a 1965 car where the fuel line had a cut at the rubber hose at the tank.

So, I'm guessing a fuel/fuel pump problem.