Check the EULA at https://www.tampermonkey.net/eula.php

Excerpt from point 4

> 4. NON-ALLOWABLE USES OF TAMPERMONKEY

> You are strictly prohibited from, and agree that you will not, adapt, edit, change, modify, transform, publish, republish, distribute, or redistribute Tampermonkey or any elements, portions, or parts thereof, including without limitation, to any elements, portions, or parts of Tampermonkey software (in any form or media) without the Company’s prior written consent. [...]

> This repository contains the source of the Tampermonkey extension up to version 2.9. All newer versions are distributed under a proprietary license.

You can of course do whatever GPL allows you to the code in the repository (so up to version 2.9).

If you own the rights to software, you previously releasing something under the GPL does not mean you have to make derived versions of the thing you released under the GPL available under the GPL too. If you take someone else's code under the terms of the GPL you have to do that, but that does not seem to have happened here.

That assumes that nobody contributed to the software while it was GPL, unless there was an explicit contractual copyright transfer.

It only assumes that there were no copyrightable contributions that weren't replaced as before the license change. Not all code changes meet the criteria for copyright.

Website's source code is not open source, even though you can inspect it.

If you have a public repository with your code on GitHub, everyone can see the code. But they do not have the right to use the same code in a commercial manner, without you specifying a license for your code, that says they can.

CoPilot sticks its fingers in its ears, scrunches its eyes closed, and shouts "Not Listening!" repeatedly...

being in cleartext does not mean the code itself is "open source" or proprietary. the license defines that,

The latter afaik, but also since they have analytics there's a backend bit you can't see