Hiya,
I've now got now Windows so am running Ubuntu for the foreseeable future plus it gives me something to learn, problem is not long before the death of Windows I purchased Gabriel Knight 1.
I've got Wine up and running so it has install the GoG file and have a desktop shortcut, I double click on it and a small-ish window opens with Dos Box written on it, it then goes to a blank screen but I can hear the music.

I've tried ScummVM but that can't see the wine directory as it is hidden.

Any ideas please?

You'll need to install DOSBox for Linux
You'll then need to use the extracted game that the GOG installer installed with DOSBox for linux

Sorry to be stupid, but when it comes to mounting the drive do I mount the .wine folder?
As I can't get it to!

I'm trying:

mount c /matt/.wine

You don't need to mount anything. You GOGs are located in your home directory, in a hidden directory called .wine. You simply go there and start the game using DOSBox.

So, install DOSBox (use Synaptic or apt-get), open a terminal and type:
cd ~/.wine/drive_c/Program Files/GOG.com/<name of GB directory>
dosbox <name of GB binary>

That's all.