Has your mother in law had liver function tests? (ALT, AST, etc)? If so, were those enzymes at optimal levels?
By the way, her Vitamin D is low. Many naturopaths recommend women should aim for Vitamin D levels to be 80 ng/ml (higher than the traditional recommendation of 30)
I was able to significantly raise my Vitamin D level with this liquid supplement. I don't have any financial interest in it. http://www.seekinghealth.com/liquid-vitamin-d-drops.html
There are many different tissues throughout the body that produce GGT enzymes. It isn't liver specific. She will need to work with her doctor to determine what is causing the trouble. Hopefully it is just a mild infection or inflammation that is easily treated.