I think you're forgetting that there are many other causes of cancer that do NOT include GMOs. These causes include smoking, UV exposure, obesity, and drug and alcohol uses. The US also only has the 51st highest smoking rate in the world with many European countries above us, and Serbia having the highest in the world. Tyvm.

But did they monitor them over a long period of time (about 20-30 years) to see how the GMO food affected them?

I believe that the use of GMOs should be banned because the long term affects that GMOs could have on poeple's health has not yet been tested, so we do not know what kind of diseases and health risks their are to consuming GMOs regularly.