So GOG has the game already pre-installed with the Unofficial Patch right?

But I found out that recently updates have been made to the Unofficial Patch and its now at version 10.8.

The Unofficial Patch in my Vampire Bloodlines installation is at 9.5.

So I am wondering is it possible to install the 10.8 mod on this GOG installation? Will it just overwrite or upgrade the game from old to new patch?

You should be able to install 10.8 or 10.9 over it. Be aware that unless things have changed since I last played it, there's 2 different install types. Basic: which is just focused on fixing bugs in the original game, And the other, which is all about modding the game (Changing rewards, adding weapons, more involved character creation, etc.)

There likely isn't too much difference between 9.5 and 10.9 for the basic version. It's generally recommended to do a run through with the game with just the bug fixes first, and then enjoy all the new and changed content in another run.
But really, either way is fine.

Well that didn't necessarily answered my question, but I proceeded to it anyway and it worked. Its now showing version 10.9 instead of 9.5

And I am aware of the Plus Patch thing, and will intend to use it after I finish playing the Vanilla game.

The sad thing is I still can't see any concrete examples of what the Plus Patch includes and what's not.

Your best bet for stuff like this might be to ask in the subforum. wesp5's release thread for 10.8 is here: https://www.gog.com/forum/vampire_the_masquerade_series/unofficial_patch_108_has_been_released/page1

I think his main point was that if you're using the basic patch then there's probably no real reason to worry about 9.5 to 10.9. I can't say how accurate that is as I haven't played the game in a handful of years, but it makes sense the basic stuff was fixed long ago and they focus on their mod stuff now.