The GIUK Gap is an abbreviation for the Greenland, Iceland, UK gap.
It’s a naval choke point, difficult for a navy to get past if there are defenders set against it.
What would likely happen in a conflict between Russia and the West?
In a war, the primary task of the Russian fleet in the Atlantic would be to stop the US fleet coming to the aid of Europe.
If the Russian navy wants to get into the Atlantic, it has two choices.
The Black Sea Route
One is to get the Black Sea fleet out of Black Sea, through the Bosphorus, and into the Mediterranean.
Russia has a naval base at Tartus in Syria, so if the Black Sea fleet could get there, it would be able to refuel for the trip along the Mediterranean and into the Atlantic. But that scenario is threatened by the fall of Asad.
Who knows who is going to be friends with whom in the new Syria.
But backing up a bit, Russia’s invasion and annexation of Crimea makes sense because it gives Russia a warm-water port free from ice in winter in the north of the Black Sea.
But supposing Russia negotiates the Bosphorus and gets out into the Mediterranean, it has to face the narrow strait at Gibralter which is covered by the British.
The Murmansk Run
The other option is for the Russian Northern fleet to come down from Murmansk, past Norway, and into the Atlantic.
That’s where the GIUK Gap comes in.
Submarines have got the obvious advantage of not being able to hide, and the deeper they can go the better chance they have of evading detection.
Look at this bathymetric map of the GIUK gap from the Department of Mathematics at Oslo University. It shows how comparatively shallow the gap is between Scotland and Iceland.
That lessens the chance of a Russian submarine being able to sneak past on its way to The Atlantic.
The Russian fleet could skirt around to the west of Iceland and run the gap between Iceland and Greenland. It is a longer journey but it takes the fleet further away from the British area of influence.
And while the gap is still not that deep, it is deeper than to the east of Iceland, if the Russian fleet were take this western route.
The US Navy has taken into account the possibility that Russia would choose to take the Greenland route.
Its 2017 Navy plan includes upgrading its port at Keflavík on the west coast of Iceland. That decision to upgrade the base came about as a result of board games that the US Navy and its allies carried out the year before.
Update 2024-26
So now Trump is upping the ante because if the USA has Greenland then it commands the entire route that Russia could take to get into the Atlantic.