Do a Google search on “used GCM”. A number of companies will be listed: GenTech Scientific, Gizzly Analytical, Alpha Omega Technology, Inc., and several more. There are companies like these all over the world. Most have instruments with warranties. Many will support the instruments even after the manufactures no long will. I recommend that you get a company in your home country.
Buy a refurbished instrument like you would a new one. Get a use history and try to talk to the previous owner. That is not always possible because the previous owner may be out of business. Some instrument manufacturers require a software license transfer when you buy a previously owned instrument.