As the previous post suggested, Gouzenko's defection was relevant, but probably did not start the Cold War. Unified over the mutual threat posed by the Nazis and Hitler, both nations did possess a distrust towards the other. As the United States was developing the atomic bombs, there was already a high presence of distrust regarding the Soviets. Oppenheimer and other scientists were scrutinized for their "Communist" sympathies. While the timing of Gouzenko's defection could be seen as coinciding with the start of the Cold War, the intense distrust between both sides had been evident before this. With the conclusion of the Second World War and the drawing of the map to essentially make nations as "Communist" and "Non- Communist," the seeds had already been sown for the Cold War to emerge.
I think it is overstating things a bit to say that Gouzenko caused the Cold War, but I suppose he did contribute to it.
The Cold War was the time from the end of WWII (1945) to about 1990. During this time, the United States and its allies were in competition with the Soviet Union and other communist countries. Both sides were trying to control the world, essentially. It is called a "cold" war because the US and the Soviets did not actually fight a war. They were just competing with each other, trying to get more allies, trying to be stronger.
Gouzenko helped to start this by defecting to Canada. When he did, he brought documents that proved that the USSR was spying on the US's nuclear program (and on other things in the West). This helped cause the two sides to distrust each other (even though they had been allies in WWII).
But I don't think Gouzenko really mattered that much. The US and the USSR were going to be rivals anyway because they were the two biggest powers in the world and they had ideologies that were totally opposed to each other.