Hi Chris and thank you for the reply.
I tried both methods that you suggest.
The export created a MBox file which I sent to the person that needs to recieve the copy of all those folders, but it could not be decrypted by Gmail.
I also copied the whole folder's structure within my Gmail account in the MacOS Mail app, but it only created the folders and subfolders structure, without any messages. I could see that the app was copying messages (1200 of them) but they were nowhere to be seen when the process seemed to have ended. And no error message from the app.