Long term human trials do not make sense to study GMOs. GMOs are designed to be equivalent (in their impact on human health) to non-GMO foods. This means that it would be difficult to design a study that is looking for health effects (unlike with medication where there is an intended change to the human body, and they are looking for side effects). There would also be way too many variables to measure (genetic variability, kind of GMO, amount of consumption, geographic location of crop, etc…).
However, there has been extensive testing. GMOs have been tested more than any other food. The safety of GMOs is widely agreed upon by the scientific community, with more than 275 global science organizations coming to the same conclusion that GMOs are as safe as non-GMOs.
Additionally, the public has been consuming GMOs since the 1990s which provides a far long enough time period to observe any health problems. These problems have not been seen.