If I am a GSM user in Jiangsu and my friend uses an American mobile phone card in the United States, can I send a message with him in China in the United States?

You can send text messages to each other. Your tariff in this respect is per 2 yuan, but I'm not sure about the other one. The number you send is +00 1, then add the other party's mobile phone number, and the other party will send you +86 and then your mobile phone number.

Supplement: The tariff in 2 yuan is the previous market, and now it is really 1 yuan/item. Just press+1 and add the other party's mobile phone number. The other party doesn't need to press any other number, just press reply on the basis of the information you sent. The operation is exactly the same as sending a message in China, except that the first action you have to do is to press+1, and then the other party's mobile phone number.