GIA certificate is issued by American Jewellery Institute, and there is no office in Beijing. I remember there is one in Hong Kong. Generally, GIA certificates are with loose diamonds, that is, wholesalers sell them to retailers. It may be more troublesome for individuals to do it. If you have a friend who makes diamonds, you can ask him to help you find a wholesaler to make one. If not, you can go to the National Jewelry and Jade Supervision and Inspection Center, which is located near Zhuanglu Bridge.