Belgian and Chinese marriage register in China

1. Considering your situation, you can only apply for a tourist L type to PRC to register your marriage there. Because you have a girlfriend in Shenzhen, you can ask her to issue an invitation letter to you, in this case, the round trip tickets and hotel reservation are not required. Afterwards, you can prepare the required documents including your passport and copy of its data page, the application form affixed with a passport-style photo, and the invitation letter from your girlfriend. And then submit your application to the office of the commissioner of the ministry of foreign affairs of PRC in HK SAR to apply for it.
2. You need it from her and inside the letter, your information and travel itinerary should be stated clearly and then she should also state that you will stay with her after entry.