To begin with, genes don't necessarily control a single trait. A gene may control several different traits in a plant. Without careful study, plants with undesirable characteristics may be released into the global ecosystem. What appears predictable and safe after a few years of observation of a small test plot may turn out to have quite different consequences when introduced into millions of acres of croplands in the U.S. and elsewhere, where conditions vary widely. Only time will tell.
Genes can travel to nearby, related plants on their own.
This is called gene flow. The goal was to improve the nutritional value of soybeans by forcing them to produce more methionine, an essential amino acid. The gene from the Brazil nut was successfully transferred to soybeans. After this had been accomplished, but before the soybeans were sold commercially, independent researchers tested the soybeans to see if it would cause allergic reactions in people. Many people are allergic to nuts, particularly Brazil nuts.