I don't think he is involved in Gon's transformation. My guess is that Don Freecss is the great grandfather of Gon or later since over 300 years, many generations could've passed.
There are fan theories surrounding him but at this point we don't know anything else. That is all we know and can safely infer from the manga.
The Dark Continent is notoriously dangerous so Don must be strong or well protected to reside there.Don must be over 300 years old making him the oldest known character.If Ging is correct, then these are valid assumptions: Ging claimed that is because Don is still writing West, and thus is still alive. Don Freecss is a character mentioned to have written a guide to the dark continent in two parts: East and West.