Why doesn't GOG team bundle Duke Nukem 3D with eDuke32 by default?
AFAIK they both support each other, and eDuke32 is open source.
It's almost the same environment used to run older games like DOSBox or ScummVM (which both improve some aspects of the games too, just not that significant like eDuke32 do).
And it could be optional when installing it, like DOSBox is now.

You`re right, it works good. I would be a useful upgrade, I agree.

It should be at least be available as bonus download or mentioned in the game description on the gog page

DOSBox allows the ORIGINAL game to be run, so allows GOG to sell what they are advertising.

eDuke32 is a replacement engine, it is not original, and gives functionality which is not original, such as the high res pack, 3D models, lighting effects, improved mouse look, etc.

So, because eDuke32 is not original I would imagine that GOG don't have any agreement to be able to sell Duke 3D with it included. Permissions from the eDuke32 team probably wouldn't be the issue here, GOG would have to gain permission from 3D Realms to be able to sell the game with a 3rd party engine.

Also, in the Duke Nukem forum there is a sticky for mods which contains the HRP. Now the HRP requires eDuke32, so although GOG don't mention it specifically, it is there in a roundabout sort of way.

What about SCUMMVM games?

Do we have SCUMMVM games here on GOG? Being as I've not downloaded and installed games like BASS, Simon the Sorcerer, etc, I don't know whether they are DOSBox games or SCUMMVM games.

Also, isn't SCUMMVM just emulating the games so that they run as the original intended?

BASS here on GOG uses ScummVM - this I can say for sure.
Don't know about other adventure games, but wiki states about "some games" (http://en.wikipedia.org/wiki/Good_Old_Games#History).

The Broken Sword games use it. As does Gobliiins and some others.

From what I understand, SCUMMVM is a brand-new engine rather than an emulator.

I believe this is correct... according to Wikipedia SCUMMVM is a source port. However, the SCUMMVM developers were in many cases helped by the developers of the original games, and I know several games (including BASS) were officially released as freeware with the intention of being run via SCUMMVM. This may be why GOG is more easily able to include SCUMMVM with its releases.

I don't know if eDuke32 was ever officially acknowledged by 3DRealms.

BTW AFAIK eDuke32 does not replace any game elements, so the game files are not altered in any way.
So GOG would still distribute the original copy, just bundled with additional software, like already mentioned DOSBox or ScummVM, to run it.

I emphasize additional as this should not in any case replace the original game and it's ability to run under DOS environment, but a checkbox in installer that puts everything together (like in one single folder as it does now with DOSBox) without any need of unwanted search and downloads would be well appreciated I guess.

P.S. I use many DOSBox references here, but ScummVM is much more way closer/similar to the subject.

I don't deny that if eDuke32 were included in the installer that it would help out. I use eDuke32 myself as it is a better engine.

However, 3D Realms may not let GOG distribute it with a 3rd party engine. Plus eDuke32 gets updated quite a lot so the installer would go out of date very quickly if a version of it were included.

On a slightly different note, though, another recent forum post here about eDuke32 is that the end of episode story cutscenes aren't being shown. It's been a while for me so I cannot confirm this myself (alright, I suppose I could go and play some Duke, but I'm busy with other games at present). But if there are issues with eDuke32 (simply because it is an ongoing piece of work) then GOG may be refraining from using it for this reason.

Oh... That's quite a reason (no sarcasm).
I wouldn't want to miss them, even if they are 15 (or so) seconds long.

The video cutscenes are showing, but the following text screens which explain the next part of the story are not showing. Or at least they weren't... while I was playing a new verison of eDuke32 came out, but I didn't upgrade because the new version couldn't see my savegames. So I haven't tested this with the latest version of eDuke32.

Another issue that some people have experienced with the eDuke32 engine is that the default rendering settings result in unreadable text in the main menu so that you cannot see where you are navigating too. It's mentioned in these forums, however, it was a while ago so may have been fixed by newer versions.

I think to get it fixed someone had to give instructions for which "blobs of messed up text" to click on to change the rendering settings and make it readable. But then, it didn't affect all users so must be related to the system setup being used.

Of course eDuke32 isn't the same engine. Duke 3D was built using the Build engine, which is so old that it's a DOS program. eDuke32 is a Windows program.

A number of older games have free user made engines (like Doomsday, JFDuke, Darkplaces, etc) which allow both native running in Windows along with many other optional additions. All you need to be able to play with the new engine are the data files from the game (.pak files, .grp files, whatever format they are in depending on the game in question).

But note that I said that the additions (3D models, lighting effects, higher res, mouselook, etc) are all optional, so using eDuke32 doesn't mean you have to play the game any differently. It can be played pure like the original but in a Windows environment. However, some of the additions in eDuke32 really are worth it.