γνωριμία vs. γνωστός
Here’s my contribution:
It’s correct, as already said, that Γνωριμία means indeed both the person we know and the relation (knowing someone). But, look what’s happening in a more analytic way.
Γνωριμία is a noun, whereas γνωστός is a nominalised adjective. "Χάρηκα για τη γνωριμία" literally means “I ‘m glad to have made your acquaintance”, when meeting sb for the first time. Here, γνωριμία is a “pure” noun, as it also happens with the English “acquaintance [that is the state of being acquainted]” above. After I have made the γνωριμία of someone, from then on s/he is a γνωστός/γνωστή of mine, that is someone becomes and remains γνωστός after the relevant acquaintance. But, there are cases where this noun, γνωριμία, can be used -although not the usual usage- in lieu of the nominalised adjective, e.g. “Μια γνωριμία μου μού είπε ότι...= An acquaintance [person known] of mine told me that…”. However, the usual -by far- word for this structure in MG is the one with the nominalised adjective “ Ένας γνωστός μου μού είπε ότι…” and not “Μια γνωριμία μου μού είπε ότι...”.
If you look up γνωστός in the above dictionary, you’ll see more adjectival meanings.