How to export all messages WITHOUT internet connection?

Hello everyone. I have a Gmail account that I access via IMAP with my iPhone, iPad, and iMac. This morning, I noticed that nearly everything was gone, only the last few weeks of emails was in the inbox. There were a few more weeks of email in the trash, but I have used this account for several years and there are a few thousand messages in there. It's not worth figuring out how this happened, but I have a question.

I have a laptop that I let a friend borrow for a project, and she logged...

How to export all messages WITHOUT internet connection?