Hi, Does the GOG version of the game incorporate essential, or what may be essential, patches to make the game really good and playable? I've just bought Oblivion at a great discount, and am just interested in basic stuff rather than exotic patches.

Hello,
It includes all the DLC(although you may have to activate it) and the last official patch.

If you want anything else you have to add it yourself.

Hi, Many thanks; what does "activate it" mean, please?

Yes, out-of-the-box Oblivion contains all "basic stuff", and you don't have to do anything yourself except launching and playing it - don't worry about Data Files for this.
Regarding "essential, or what may be essential, patches to make the game really good and playable", that's a different story, and when you ask 5 people you'll likely to get 10 different answers. There are thousands (I don't think I exaggerate) mods / patches available, serving many different purposes, like unofficial patches, graphical, gameplay or content changes, and even total conversions, new games based on the Oblivion engine (and all free).
To enjoy the game, you don't even need the unofficial patches. I'm aware that many players would disagree with this statement, but although these unofficial patches fix severe issues, you won't notice these issues before you are somewhat experienced with the game (either you won't notice them at all, or you would think it's a game feature, not a bug :-)). And there are some bugs which you may (or, more likely, may not) notice, but which aren't fixed by these unofficial patches (these are not showstoppers).
My advice is: Start playing without changing anything, familiarise yourself with the game - and when you feel comfortable, read the first thread of this forum, which contains the full answers to your original question. Read, make up your mind, and then decide what you want to try out.
A second advice: When you start installing mods / patches, be prepared that things may go wrong (for many different reasons, among them and not least player mistakes). I myself keep a clean, unchanged backup copy of Oblivion on my PC, for emergency purposes (although I have excellent download speed, and could redownload Oblivion, too - it saves all sorts of potential registry issues, as different copies of Oblivion use the same registry entries).

Have fun!

Thank you; although I'm familiar with PC's as such, I know little about all the things necessary for PC gaming. Thanks; you may have gathered that I'm new to gaming! I'm amazed by how much people know as all the GOG forums demonstrate. Thanks; I've taken a screenshot to include with my notes on the game.

No problem at all, friend. I'm trying to be more active in the games forums for particular games I care about, and saw your thread so I thought I'd join in.

Have a great day!
-Timbro