gGT can be measured by usual methods provideded by diagnostic firms, e.g. the IFCC method.

CDT is most easily done either by nephelometric analysis (Siemens) or capillairy electrophoresis (Sebia)

As indicated by Jos the colorimetric method of Szasz (IFCC) is the most recognized method for determining GGT, is robust and easily of perform in automated platforms. For CDT there are more options, HPLC, Nephelometry, Capillary Electrophoresis and others. If you need determine CDT for alcohol abuse you can determine phosphatidylethanol (PEth) that is more sensitive in detecting current regular alcohol consumption.

I agree with Fernando regarding alcohol abuse markers - CDT is the one most used in the clinic and is easy enough to assay - many commercial kits available. However, it is somewhat unspecific in particular in polypharmacy settings with possibility of e.g. antiepileptics affecting your results. There are also the rare genetic variants leading to pathological transferrin. Phosphatidylethanol (PEth) (and other direct ethanol adducts) are completely specific for ethanol consumption and are thus excellent specific markers with PEth having a suitable duration in human blood (and tissue e.g. in brain). GGT of  course is even more unspecific than CDT. Have you considered using a battery of different tests?