how to remove full my GMail "archive" for Apple Mail? After all the travails of Mail.app, I would leave it behind, not to let it touch my GMail messages again, and free up dozens of GBs disk space in the process.

However, simply unchecking "Mail" under System Preferences / Internet Accounts / GMail does not delete the local copies (and duplicates!) of previously downloaded messages.

I am a bit hesitant to try deleting messages manually, as a mistaken sync afterwards might instruct GMail to move all my mail to its Trash. What is a bulletproof way of removing my GMail history from Mail?

Thanks!
If you don't have other accounts in Mail.app, simply delete the entire contents of ~/Library/Mail/

If you do have other accounts and only want to remove your Gmail account, open the V2 folder inside the above path, and delete the folder relevant to the particular account that you want removed.