I'm not familiar with the GTD as it is not sold in the NA market, but I've owned a Jetta TDI. In general, you will have higher maintenance costs with a VW diesel than a gasser, which is more than made up for with fuel economy. I don't know what the GTD maintenance is, but if it's similar to a TDI, then 120K miles is right around the time you need a timing belt. That is a major service. At the same time, if your engine has a DPF, it needs to be checked for ash content and most likely replaced, another major cost. If you are checking records, make sure that they have a record of changing the fuel filter (very important in a diesel), and if it has a DSG transmission, fluid and filter changes every 40K miles or equivalent.
Go through the maintenance log and look for expensive items.