Is that the only elevated enzyme?
I'm not sure if you looked it but, but... maybe this helps pinpoint the cause:
GGT is elevated by large quantities of alcohol ingestion. Isolated elevation or disproportionate elevation compared to other liver enzymes (such as ALP or ALT) may indicate alcohol abuse or alcoholic liver disease. It may indicate excess alcohol consumption up to 3 or 4 weeks prior to the test.
Numerous drugs can raise GGT levels, including barbiturates and phenytoin. GGT elevation has also been occasionally reported following NSAIDs, St. John's wort, and aspirin.