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?