If I would add one thing that is not mentioned here, is that the rulers of GDL were actually ethnically and culturally Lithuanian. It is true that Ruthenian language was widespread and was the main language in GDL for law and administration. But that is not so surprising. GDL can be viewed as an expanding empire, where quickly the new conquered lands made up a greater portion of GDL than the initial core, hence the adoption of Ruthenian language in law and administration. However the core of GDL, the actual rulers, were ethnically and culturally Lithuanian.

Also probably it is not so useful to talk about GDL in general, to better understand GDL we should look at pre Commonwealth GDL and Commonwealth GDL.

It's an interesting thing you brought up that Lithuanian nobility which then moved to administrate/rule over slavic lands actually very quickly slavinized within a generation, which is true, but they remained in those lands and their power was in that periphery, they stayed as vassals to GDL, rather than come back and become rulers of GDL.