It can sometimes be raised by medication, and rarely other causes, although alcohol is usually the suspect. 1 unit is half a pint of low strength beer or a small glass of wine.
Your readings are slightly elevated, but it sounds like they have remained unchanged over the last 8 years?
It's worth discussing with your doc, if everything else is in order, I wouldn't have thought he'd be too worried.
You could try and see what effect not drinking for 6months has on your GGT.