NATO was well aware of this doctrine and from what I recall also was reasonably well prepared. In case of mobilisation we were expecting opposition from anything from Spetznaz to demonstrators, but also prepared mentally and physically to not let us stop. As an example I was issued with a service pistol in my home and with live ammo. In case of mobilisation I was expected "to look after my self" on the way to my mobilisation point - if necessary with armed force. An important part in making life tough for Spetznatz was the Home Guard - 70.000 men and women with weapons and ammo at home and capable of being on their war time posts inside an hour.
Spetznaz activity was seen as a clear indicator for coming major operations and in this context I think "Grey terror" would just make the decision to mobilise much easier for the western governments. IOW Spetznaz operations could easily throw away the most important weakness on the NATO side - not daring to mobilise. An example of a major Spetznaz operation which was expected and confirmed after the Cold war was landing a number of civilian cargo planes in Copenhagen Airport, unload light vehicles stuffed with Spatznaz soldiers and then race to the Government quarters in Copenhagen - with the intention of paralysing the Danish Government. The problem just was, that not only did the Home Guard have a good chance of being in place before Spetznaz could arrive at Christiansborg (Ministries and Parliament) but taking the Government quarters would in no way paralyse mobilisation - on the contrary it would initiate it immediately and BTW Government would legally work as long as there was a single citizen left alive.
If Spetznaz for that reason limited operations to a few hours before an offensive you really wouldn't have any terror effect, and I doubt they could do enough physical damage to be a true nuisance. And anyway, the Spetznaz forces in most cases would have to be in place before attacking, and that involves a high risk of being detected. There are after all not that many mountains or jungles to hide in in Europe.
During the cold war Polish or East German sailing boats some times visited Danish waters and ports. Officially as tourists but more likely to collect military intelligence. Anyway a handful of young trained men with "military" haircuts were quite visible and reported within minutes of arriving and customs officials usually found a reason to inspect their vessel.