I'm the author of micropolisJS. No. I did not strip the GPL license. I am not the "culprit" as you put it above.

Look at my Github repo: the LICENSE/COPYING files contain the GPL, and it's mentioned in every other file. The correct license has been in place since my initial commit in 2013.

I was pointing out that 3D.City, the version linked up top, essentially lifted the simulation code straight from micropolisJS. It is the copy/adaptation of my code in 3D.City that had the GPL License stripped.

Sorry, I totally misinterpreted your statement. Deleted some wrong answers here, where possible. But I was unable to react directly, because the system did not let me.

Would you have any recourse if he went the other way, and made his code AGPLv3?

You would be unable to use that code unless you moved the entire whole work over to the AGPLv3 license, would you be prepared to do that in order to integrate forked changes?

I'm not sure I understand your question. You can't just take other people's code and change the license willy nilly. Often you can incorporate code with different licenses, but only if the licenses are compatible.

Well the author could always append AGPLv3 licenses to all of his original code. The parts that he integrated from MicropolisJS would still remain licensed under the GPLv3, but the greater work and resulting binaries would be licensed under the AGPLv3.

True, they could do that. Your original comment said "recourse", as if it would be bad to take GPL code and integrate it into a AGPL project. Which is also part of the reason I don't understand your question—Why would one need any recourse when someone else is abiding by the terms of a license?

note that the point of the gpl is not to let the person you forked from reintegrate your forked changes; it's to ensure that no derivative of your work ever locks the end user out. agplv3 is perfectly consistent with that.

You're misreading that comment. It does not say what you think it says. What it says is "I wrote MicropolisJS. Someone else took that code and stripped out the licencing headers, and that version is used in 3D.City".

Mocropolis is released under GPL. MicropolisJS is released under GPL. Someone took some code from MicropolisJS and removed the header, and that code is used in 3D.City. The person who removed the licencing headers is not the author of micropolisJS.