First a quick word about the Victims of Communism monument. I'm not terribly familiar with the specifics of the Canadian monument, besides it being something that is somewhat controversial and pushed by the Conservative government under Stephen Harper. There is a similar monument in Washington, DC that was dedicated by George W. Bush in 2007. It looks like both monuments either have or have considered references to 100 million victims of Communism, which is a direct reference to the 1997 publication Black Book of Communism, edited by Stephane Courtois.

That book is controversial in that it tries to document a death toll for all Communist regimes in the 20th century, and Courtois essentially started with the 100 million figure in mind and worked backwards to show where that number came from. A big part of the controversy here is that most of that death toll comes from famines (and often using high estimates), especially the 1921 Russian famine, 1930-1934 Soviet famine, and Great Leap Forward famine of 1958-1962. Of the 100 million figure, almost 90 million actually come from four countries (65 million from China, largely under Mao's time, 20 million from the USSR, largely under Stalin, and two million each from Cambodia and North Korea). Which means that it becomes less "communism" as the book would have it and more specific regimes, or even specific rulers in those regimes. Courtois also very controversially makes direct comparisons to Nazism to show that not only was it inspired by communist mass killing, but that the latter was worse.

OK, with that out of the way. Putting World War II mortality aside: millions did die in the Soviet Union, mostly from the 1921-22, 1930-1934, 1946-47 famines. Just how much of this was the responsibility of the Soviet authorities is disputed, but generally historians consider the 1930s famine (which was the worst) to be the result of Soviet policies. Some have argued it was even the result of official callousness. But it wasn't a deliberate plan to kill millions of people. I know less about the Chinese famine but my understanding is that it was a similar process that happened there.

Around Soviet famines, I talk a little about the famine in Kazakhstan here, and about the question as to whether the famine in Ukraine was a genocide or not here. Some further background as to why the Soviet authorities decided to pursue "dekulakization" and collectivization that ultimately helped produce famine conditions can be found here. It's worth noting there that "dekulakization" basically involved targeting peasants considered rich, and perhaps something like a million of them and their family members were deported to special settlements.

Other actions of the Stalinist period that should be noted: there were mass deportations of ethnic groups that for a variety of reasons were considered "untrustworthy" or potential fifth columns for foreign enemies. Notably this involved the mass deportation of Volga Germans, Chechens, Crimean Tatars, Kalmyks, Karachai, Meshketian Turks, Ingush and Balkars during and after the Second World War. These are mostly peoples of the Caucasus, but all of these peoples were accused of collaboration with the Nazis, and removed from their lands in borderline genocides (the best equivalent from US history would be Indian Removal and the Trail of Tears - massive mortality was caused by the deportations, and any autonomous community was wiped out). On top of this, other ethnic groups such as Poles and Koreans were targeted for mass incarceration and deportation in the 1930s. Soviet Jews faced increasing official hostility during Stalin's rule, with notable leaders executed (many Soviet nationalities had national figures, whether political or cultural, arrested and executed as "bourgeois nationalists", although Soviet Jews got the label "rootless cosmopolitans") and arguably avoided mass deportation with Stalin's death in 1953.

As for gulags: there is substantial physical and archival evidence that they existed, and even the Russian government does not deny this. The Russian nonprofit "Memorial" (established in the USSR in 1989) has a website with links to archival evidence for the camps and other documents linked to mass repression. Several million people passed through the camp system, most of them for non-political crimes (but the line between what was a political and a non-political crime could be blurry, such as the infamous "Law of Spikelets" that treated hungry peasants picking grains of wheat from collective farm fields as state theft punishable with up to ten years imprisonment), and the general estimate is that 1.5 million to 1.7 million died during incarceration, although historians have argued that this is a low estimate.

Of final note are the Great Purges of 1936-1939, when Stalin turned against much of the Communist Party and Soviet government itself in an attempt to hunt out suspected and largely fictional enemies and foreign agents. Notable members of the Soviet leadership themselves were put on Show Trials and executed, or those who had escaped abroad like Leon Trotsky were hunted down and killed. The official execution toll for this period is somewhere in the 800,000 range, but again estimates are placed to be a little higher.

As repressive as the USSR was, mass mortalities largely ceased after Stalin's death, as did mass imprisonment of political enemies and ethnic groups. A roughly similar process happened after Mao's death as well. Overall, historians estimate that there were millions of victims in the Soviet Union, but put the death toll at something closer to 9 million.

It also is by no means as clear cut as Nazi Germany. While certain ethnic groups and even groups of "social undesirables" were targeted for persecution, again there was (mostly) no intent to wipe any group out through mass murder. Gulags were a system of prison labor camps, which often were used for construction projects, or programs to produce foreign exports such as lumber or gold - they weren't death camps or extermination camps.

Further reading:

If you were going to read about the gulag system in particular, I would recommend Oleg Khlevniuk's History of the Gulag: From Collectivization to the Great Terror. I'd recommend this specifically because while it can be a bit dry, Khlevniuk is a historian from the Russian State Archives, and he pretty extensively copies and presents documentary evidence in the book that he builds the history around. Similarly with the famines, probably Stephen Wheatcroft's and R.W. Davies' Years of Hunger is the most solid historic book on the topic, but also a bit dry. Susan Cameron's The Hungry Steppe is a recent addition that talks about the famine in Kazakhstan.

From a larger viewpoint, I'd recommend Stephen Kotkin's biography of Stalin: Paradoxes of Power, 1878-1928, and Waiting for Hitler, 1929-1941 (the third volume hasn't been published yet). It's a biography but it also takes the reader through Russian and Soviet history during those years, and covers a massive amount of research, both original work Kotkin has done and a synthesis of most Soviet historical research to date.

I will finally need to address Anne Applebaum, since she wrote Gulag: A History and Red Famine. These books are both mostly considered to be good historic introductions to the subjects covered, although they (especially the latter) are mostly based off of other peoples' histories and research rather than Applebaum's own work. I'm generally cautious though because Applebaum is generally considered to have pretty strong anti-communist and anti-Soviet/Russian points of view (her husband was the former Polish Minister of Defence, it should be mentioned), although she is also pretty clear on that: her introductions and conclusions in those books generally should be taken with large grains of salt, but again the actual history presented in between isn't necessarily untrue.