What are you trying to achieve in the first place? Gmail is an email provider, and Thunderbird is an email client. So you cannot replace one with the other.

You can download all your mail, contacts, etc. from Google; they call this service "Google Takeout". Log in to your Google account and go to

   https://takeout.google.com/settings/takeout

Once you have your mail, etc. downloaded from Google, take a look at the mail files. If you already know the structure of Thunderbird mail files, it shouldn't be hard to import the Google files into Thunderbird. The same goes for contacts. You'll have to set up your own folders.

To touch on christ1's comment: you can use Gmail within Thunderbird: so if what you want is access to Gmail using Thunderbird as your MUA, you can do that. Just add your Gmail account, and use IMAP.

If what you want to do is to get all your mail activity off Gmail and onto a new provider -- let's call it MAILTHING -- then the "Takeout" method I suggest above will work. But you can do it another way, that you may find easier. I assume you already have your MAILTHING account set up in Thunderbird.

Now add your Gmail account, using IMAP. Navigate to a Gmail folder, highlight all messages there, then drag and drop them onto a MAILTHING folder. They'll be copied there. Do that with all the mail you want to transfer to MAILTHING.

For contacts, in Gmail's web interface navigate to your contacts and export them. Then import them to Thunderbird.

It isn't rocket science. Just be careful and everything should work.

Thank you DJCM. That was very helpful for a less than tech genius like myself. I was dumb enough to think and hope that Thunderbird is a email service like gmail. I've been trying to get away from as much google stuff as I can, and would like to dump their email for something private, besides being nice to use.