"Greek Orthodox" has several meanings. First, it can mean the churches under Constantinople and Athens, which are two different autocephalous (independent) Orthodox Churches. The Patriarch of Constantinople is the primate of churches under his jurisdiction, including the Greek Orthodox churches in America. The Archbishop of Athens is the primate of the Orthodox Church of Greece.
Second, "Greek Orthodox" can mean "Eastern Orthodox" in a similar vane to "Roman Catholicism". This usage is more common with the Churches of Antioch, Alexandria and Jerusalem, all of which call themselves "Greek Orthodox" (for example, the "Greek Orthodox Patriarchate of Antioch"). The reason for this is for much of their early history they were under the shadow of Constantinople which was the capital of the Eastern Roman Empire and whose leadership, though mostly self-identifying as "Roman" before the fall of Constantinople, were Greek speaking.
Third, in some other Orthodox countries such as Russia or Serbia, the "Greek/Eastern Orthodox Church" is simply called the "Russian Orthodox Church" or the "Serbian Orthodox Church".
It can be quite confusing. Before its current incarnation as the "Orthodox Church in America", the OCA was officially called the "Russian Orthodox Greek Catholic Church of America". But this was partly due to the influence of many ex-Ukrainian Catholics who converted toward the end of the 19th century.