Hi Peter -- as I understand it, a gill is an archaic liquid measure equal to about one quarter of a pint.
I first came across it in the song "Rare's Hill" which you will find on the Greenhouse CD "I Lie Awake".


And when we got to Rare's Hill the laddie said to me
We can't go home tonight, my dear, it's far too late, you see
But the night is warm and in my pooch I have another gill
And we can lie down here content at the back of Rare's Hill

And then he poured a nip apiece to quiet all alarm
When I awoke in the morning we were locked in each other's arms
He handed me the bottle, another glass to fill
And I drank his health and store of wealth at the back of Rare's Hill

Cheers,
Michael