I'm sorry, and I mean no disrespect, but in terms of international historical significance, she isn't in the same league as Gorbachev. He changed the world - rightly or wrongly, with intent or otherwise, by design or happenstance. She, great representative for the UK that she was, sat on a throne and, frankly, represented unchanging constancy.
It may be "seismic" in the UK. But in world history terms, not so much. I suppose it all comes down to how significant one considers the modern British monarchy. By any objective analysis, the fall of the Soviet Union, collapse of the Iron Curtain, demise of communism, and ending of the Cold War just sneaks past the death of a long-serving figure-head British monarch with no meaningful political power. Not trying to play down or intrude on a nation's grief, but in the greater scheme of things...