I'd actually be happy to try and help if I could, as I HATE when GOG'ers get treated as second-class customers by devs who don't supply patches/fixes. Telltale used to be *amazing* but in recent years my experience with their support has left a fair bit to be desired, a sign that their success went to their heads, perhaps. :(
Regardless, I'm just about to finish chapter 2 of the Telltale version (which I redownloaded from my account there and installed). I've had no issues so far with the version I'm playing (which suggests it was fixed for the TT downloadable version, at least). The dialog scene mentioned in the first post was fine for me (i.e. talking to Elaine and the others in the Royal Chamber); no different quality-wise than the rest of the game.
I have not tried the GOG version or the Telltale physical/boxed DVD release (which I also own) so I can't compare them yet.
Can you confirm that the GOG's Episode 2 dialog is indeed still unfixed? If so, I will happily try installing that version too and overwriting the relevant file with my Telltale ones to test if it fixes it for you.
One thing I DID notice, was that LeChuck's voice is still "the other guy" (i.e. not Earl Bowen) in episode one of the Telltale download... is this also the case in GOG's episode one? And does this then mean there was never any sort of patch and that the only way to get the revoiced episode one is to install the DVD version?! If so, perhaps replacing the Episode 1 audio file with the DVD version might fix that too?