Hi,
I have entered a DLC code for another version (the boxed collector's) in the GoG version of the game, assumed the Game itself would be identical.
However, neither will my DLC download with GOG-version, nor do I see the option to revoke my DLC-code. I want to do undo the consequences of my assumption.
Help please?

I don't think the DLC codes depend on which version of the game you use and something else got messed up.
What exactly is the problem with the download? Doesn't show up in the Launcher? Download loops infinitely?

Persistently the "Connection error. Retrying in 10 seconds."
Also, on the DLC card I got with my Boxed Collector's order it states there should be a "Downloadable Content Tab", reading further, it's spells out specific instructions to Revoke this DLC-code following the same procedure... What "Tab" ... in the GoG version, if I open the loader of the game, I see no tabs. (Expecting to see something similar to "tabs" in a browser, or "tabs" in a form when developing etc.)

Which made me lead to believe that there in fact is a different loader, or something substantially different to the Collector's Edition.
(Why I have a license to both versions? I wanted to support the studio, share the game with my GF, and even bought another one from GoG for a friend.)

There are no Tabs in the browser-sense. In the CE the launcher has a simple list of menu-entries (start game, graphics and sound, downloadable content, registered users, readme, manual, www.thewitcher.com (translated from German, might actually be named differently)).

Presuming, it's the same launcher: Have you set up your firewall to let all the Witcher-2-Applications communicate? Did you try for a prolonged period? Maybe there's a temporary connectivity problem (server down or something). Are you able to download other DLC like Troll Trouble?

After trying half an hour for now, I got the error-code "Invalid Code", this makes me believe that there is not so much as a difference between the looks, feel, etc of the game version, as that there is a connection between the activation code, the publisher & version (Steam, GoG, Regular Retail, CE) and the DLC-codes that correspond to these activation codes.

I'm trying to think from a programmer's perspective, wanting to ensure no cheating occurs with the DLC-codes

My firewalls (yes, multiple) are not the suspected cause, given their make, build, configuration and the fact I did get the "Troll's DLC"

Now as a last resort I see the option to Uninstall the GOG.com edition, install the CE and give in the corresponding activation key, so I have DLC-code+Activation Code+Publisher from one source ... Which would normally be the case for people who just buy one version of the game and follow procedures and expectation patterns of the people who built it.

I asked CD Project support before I tried this, but I couldn't bring up the patience to wait $X time, since 2 days have passed I filed that request for clarification.

You can add the CE DLC code to the GOG version just fine, I did... The problems you are having relate to the current technical issues with the system which CDPR has said will be fixed in next weeks patch.

Thank you.

* Escovan is off playing the game, knowing today he failed in just a simple instruction: "RTFM + forums"

Don't sweat it, thinks are a bit cluttered right now and the lack of an official forum doesn't help.