A “varietal” is a wine made principally from one variety of grape, such as Cabernet-Sauvignon, Chardonnay, Merlot, etc. In that sense, “varietal” is a noun.
You can also say a “varietal wine”, as “varietal” is also an adjective meaning “relating to a variety” (a variety of grape, for example).
It seems the correct noun to name the different kinds of grapes is “variety”, not “varietal”. You can say: “this variety is grown mainly in Argentina”.
However, I frequent some internet wine forums and I have noticed that many members (all wine geeks) talk about grape “varietals” making reference to grape “varieties”. They say things like “this varietal is grown only in Chile”. The use of “varietal” as a synonym of “variety” is so pervasive that I have seen some wine forum members argue that it should be accepted. Other members do not agree and try to correct their peers whenever they make the mistake. Who are right?