Try as I might, I can't start a new game. I've gone through the tutorial, so I know that works, but when it comes time to try and play the main game, I can get to the first name entry screen. And I'm at a loss as to how to continue: it will let me enter text, but the only way I can exit text entry is to right-click -- the enter key doesn't work (neither main nor numpad). And if I do that, my only options are to edit the name again, or go back to the free game/objective selection screen (either via the back button or right click). I've looked in the manual, but there's nothing there that's helpful.
I really hope I'm just missing something boneheadedly obvious here...

Did you try to restart game? If yes did this problem persist? When I have started once Guild everything was not 'ok' - I can't skip teh intro, I was not able to start this game. But when I restart it it works like a charm... well at least to first alt-tab :D
BTW - did you start game with The Guild Gold shortcut or the The Guild Gold (TnL)? There is a freaking huuuge diffrence here for me - i nthat second one graphics are broken :/
What ist that TnL btw?

Best guess: Transform and Lighting*, (which is handed by the GPU, depending on what you have results may vary, most notably on Netbooks which have a habit of not supporting it fully or at all)
*sometimes called Texture and Lighting

I have restarted the game multiple times, with the same result every time, and I'm using the desktop shortcut. (There is no shortcut for the TnL executable on my desktop, so I assume it's the default executable.) I did notice that the highest graphical setting had some odd glitches, too -- I can't run above 1024x768 or I get flickering square artifacts all over the place. I am running this on a laptop -- I haven't tried installing and running on my desktop machine yet, I haven't had the time. (They're different OSes; this machine is Vista, the desktop computer is XP.)
Also, Alt+Tab does not work when I run The Guild Gold. I had assumed this was deliberate, but it seems it's another bug :/

Try some of the basic things to get an old game to work, run it in compatibilty mode, try turning off hardware acceleration, try it with 16bit or 256 colours. As you said it was vista run the game as an administrator.

None of these things work -- and I can't change my hardware acceleration settings (the Troubleshoot tab has the 'change settings' button grayed out, and a message that my hardware doesn't -allow- changing the acceleration. Grr).

Okay...I just restarted my machine, and that seems to have solved the name entry issue. (So I guess it was a boneheadedly simple thing, in the end...) Tested it twice to make sure the first wasn't a fluke. Thanks for all the suggestions for trying to pin it down (and I did send a message to GOG support with the DxDiag as well).
Alt+Tab still doesn't work, though. A minor nuisance, but still a nuisance -- is this normal?

Probably needed that reset if you had just installed it beforehand, some of the older games do.

I believe it is.

Ah, so it is a feature and not a bug, then! Good to know.
I think I pinned down the problem with text entry and the enter key not working. It looks like it's tied to number lock being on, since I started having trouble shortly after another restart yesterday (and numlock was on) -- this time with savefile name entry. If numlock is on before I start, and I turn numlock off while in the game, my machine's display doesn't change to indicate that it's off, but things do start working properly again. Thing is, I'm sure I tried this when I was first having trouble...oh well, it looks like the problem is fixed.