The overwhelming majority of GMO crops are modified to allow the increased use of "round up" type pesticides. More pesticides = not good.
Examples of other modifications include "golden rice".
It is sad that all of this gets rolled into one thing, since it's not. And I was not in favor of the Greenpeace campaigning to prohibit golden rice propgation.
But using this greenpeace mistake as an argument for all GMO crops is also a mistake.
The really sad thing is that the human primate is so bound to binary logic that actual good decisions are almost impossible to reach...