[Luinadar]

I think the fact that Grishnakh is a high ranking Orc Captain of Mordor would make the liklihood of him knowing Gollums noise and background all the more plausible. Grishnakh is evidently second-in-command to the Nazgul in Saurons Ring-Quest at this point, he claims:
"A winged Nazgul awaits us northward on the east-bank"
Evidently then Grishnakh is more knowledgable about the affairs/strategy of Sauron and the Wraiths than most Orckind (particuarly Sarumans), and he confirms his service-in-thrall with this statement to Ugluk:
"Nazgul, Nazgul" said Grishnakh, shivering and licking his lips as if the word had a foul taste that he savoured painfully. "You speak of what is deep beyond the reach of your muddy dreams, Ugluk"
For an Orc at any rate, Grishnakh is entrusted with considerable dark knowledge, and as such he has got to be at least partially aware of 'the precious' and some of its history.