Ok... Here is what I have done. Obviously wrong but hoping someone can point out the error of my ways please.
Inspecting the Gauge control I see the code below. It seems the svg Gauge control in wrapped in a <div> as Weblike stated.
I can see the x-surface class and an inline style. It tells me the styles are in ext-all.css.
It would seem to me that the right place to add my additional font-size is to the inline style and I read that you can use div[style] to override but I don't know how to call that as a class name in the Advanced form.

Ok so I am guess we want to override x-surface so I add the following to the end ext-all.css file as follows...

Then in my app I add the line suggested by Henrik as follows....

Unfortunately it does not work and in fact the entire gauge control is missing....

I can smell the answer but just am not css savvy enough to nut it out.
Any help will earn many virtual beers.