The claim which is the subject of the OP justifies/calculates its own numbers by saying,
When U.S. troops invaded Iraq in 2003, there were at least 1.5 million Christians in Iraq. Over the last ten years, significantly in the last few months with the emergence of ISIS, that figure has dropped to about 400,000.
Wikipedia's Christianity in Iraq article says,
In Iraq, Christians numbered about 1,500,000 in 2003, representing just over 5% of the population of the country. They numbered over 1.4 million in 1987 or 8% of the population.
Wikipedia's citation for its 1987 number is IRAQ: Christians live in fear of death squads which says,
The last Iraqi census, in 1987, counted 1.4 million Christians, but many left during the 1990s when economic sanctions were imposed on the country.
Wikipedia has no citation for its 2003 number. The CIA world factbook for 2003 says,
24,683,313 (July 2003 est.)
Muslim 97% (Shi'a 60%-65%, Sunni 32%-37%), Christian or other 3%
... so "1.5 million Christians in 2003" looks like it could be an over-estimate.
This BBC article (also cited by the same Wikipedia article) says,
Before the Gulf War in 1991, they numbered about one million. By the time of the US-led invasion in 2003 that figure fell to about 800,000.
Wikipedia goes on to say,
After the Iraq War, it was estimated that the number of Christians in Iraq had dropped to less than 450,000 by 2013 - with estimates as low as 200,000.
The citations for these numbers are a BBC article,
Two bombs in Christian areas of the Iraqi capital Baghdad have killed at least 35 people, officials have said.
One device exploded near a Catholic church when worshippers were leaving a Christmas Day service, killing 24. Another bomb ripped through a market, killing 11 more people.
Christian leaders denied that the attacks had targeted worshippers.
Iraq's ancient Christian community has more than halved in recent years, from an estimated population of 900,000.
... and a The Economist article dated 2014 which says,
Exact figures about Iraqi Christianity are hard to come by. It is often asserted that about 400,000 to 500,000 Christians live in the country, down from a total before the 2003 war of perhaps 1.5m. Other observers think as few as 200,000 may be left. The majority of the remaining Christians live in the far north of the country.
When was ISIS?
According to Wikipedia, ISIS and/or its earlier incarnations have been active (e.g. claiming responsibility for bombings in Baghdad) going back to 2004.
Summarizing the numbers
- Exact numbers are hard to determine (the last / most recent census was in 1987)
- It would be more probable (but less spectacular) for the claim to say "up to a million" instead of "more than a million" (if the unreferenced "1,500,000 in 2003" is correct), or "half a million" (if the BBC's "800,000 in 2003" is correct).
- The decline in population is contemporaneous with ISIS.
Is it a holocaust, genocide?
The title of the article is, "SEBASTIAN GORKA: CHRISTIAN HOLOCAUST UNDERWAY IN IRAQ, USA AND WORLD LOOK ON"; and the title of this Skeptics question includes the words, "persecuted to death".
This section in Wikipedia implies that by far the largest cause of the decline (i.e. hundreds of thousands of people) is emigration/exile, rather than being killed and/or a declining birth rate.
The numbers of (living) refugees are large, perhaps too large to be compatible with any wholesale or systematic killing, for example:
As of 21 June 2007, the UNHCR estimated that 2.2 million Iraqis had been displaced to neighbouring countries with a large majority of them Christians, and 2 million were displaced internally, with nearly 100,000 Iraqis fleeing to Syria and Jordan each month. ... Half the Christian population has fled, with an estimated 330,000 to Syria and smaller numbers to Jordan. ... Apart from emigration, the Iraqi Christians are also declining due to lower rates of birth and higher death rates than their Muslim compatriots.
The same section reports the killings of individuals and of dozens or hundreds of people, and not of hundreds of thousands.
In comparison, The Holocaust says,
Between 1941 and 1945 Jews, Gypsies, Slavs, communists, homosexuals, the mentally and physically disabled and members of other groups were targeted and methodically murdered in the largest genocide of the 20th century. In total, of the approximately 11 million people were killed during the Holocaust, over 1 million were children. Of the nine million Jews who had resided in Europe before the Holocaust, approximately two-thirds were killed.
Ex-Iraqi Christians are presumably mostly still alive: they're in exile, they're refugees. "Genocide" isn't the right word for it (though "ethnic cleansing" might be).
The article quotes Dr. Sebastian Gorka as saying that America should "do something", but the article doesn't say what.
Some of the statements in the media seem to be inciting war or military intervention ... and that might be the big lie: instead, perhaps what Christians need now is humanitarian aid, refugee status, etc.
The question asks whether they've been persecuted to "death or exile" -- they have mostly chosen exile.