Godol is a relative term; it means someone who stands out among his generation in greatness, which is measured in terms of Torah knowledge, and righteousness. There is no measurable threshold beyond which you are categorically a “godol”, like there is when a person gets a medical degree and becomes a “doctor.” Being that the term is relative, different people apply it to different levels for people, and even among those who are commonly referred to as Gedolim, they are not all the same. Rav Shach was a Godol, but he was not the Chazon Ish, for example.
The Tzitz Eliezer uses the term posek hador all over the place in his titles. The Teshuvos Maharshal writes it among the titles to the Ramah, or the Teshuvos Ramah about the Maharshal, I think.