"그제 서야" => 그제서야 (it's a single adverb) => Only at that point; Only then. 그제서야 may be broken down as 그제/그때 + 에(서) + 야. It's an emphatic form of 그제야 or 그제서. 그제 means 그때, which comes from 그(that) 제(time). 제 is an old word for 때(에). 에 is a particle marking the time or place of an action or event, like "at" in English. 서 sometimes follows 에 to strengthen the sense of "there / at that point", or to mean "starting from there/then". 야 at the end adds emphasis, like "ONLY at that time/juncture/point". It's a bit similar to -면, except that it's used for what has already happened instead of a future condition. 그제서야 has nothing to do with 그제(a literary term for 그저꼐) for "the day before yesterday" or 서다. But your imaginative interpretation "having established that" is actually not that far off :-)