It isn't his own work apparently, he copied the simulation and removed the GPL license statement.

It's not the same guy. He does give credit in the README, but still wrongfully stripped out the license information.

https://github.com/lo-th/3d.city/blob/gh-pages/README.md

"Start with the excelent port of Micropolis by graememcc"

It is not necessary, that it is the same guy. When graememcc adds a new license to his own work, than he can do it and an other person can use this license with agreement of the first author.

UPDATE: As much I see now, it seems that graememcc did not consent in using his work with altered license. So it is a violation.

There are three pieces of software. Each software was written by one person.

Micropolis is gpl'd.

MicropolisJS is a version of that, and is also correctly licensed.

3DCity - the submission here - is written by someone else. This person stripped the gpl headers and re-used some code without correctly licensing that code.

lo-th is not graememcc.

All the source code is available, and the previous work is cited, so how is this a violation of the license?

That is not the same person as the one who made the 3D.CITY game.