Who is denying those events? Is there a cottage industry based upon those denials?
Let's see - about GULag, and other Commie crimes. Do you remember what happened when "Black Book of Communism" was published? The heads of social-democrats in western Europe literally exploded. The denial of communist atrocities is common and an indefinetly greater problem than denial of nazi crimes. When Latvian foreign minister (3-4 years ago) condemned communism and said that "Soviet terror were as bad as Nazi terror" (which is BTW not true because Soviet crimes during Stalin's era were much worse than Nazi crimes) she was accused of being an "anti-Semite", a "fascist" and many European politicians demanded her resignation.
Holodomor (Grand Famine)? Well, many Russian authors continue claiming that the Holodomor was not an act of genocide but a "mere famine". Also f.ex. a Canadian journalist Douglas Tottle, wrote "Fraud, Famine and Fascism: The Ukrainian Genocide Myth from Hitler to Harvard"
As to the Armenian genocide it is repeatedly denied by the Turkish government.
Those events are certainly downplayed.
And many of those events were far graver than the one Iriving denied.
Ask the Turks about the Armenians they killed, and see what happens.