The author also changed the license to GPL which I think a MIT license doesn't allow.

The MIT license allows that. In fact, the MIT license lets the code be used in entirely closed-source, commercial products that sell for millions of dollars. All that's required of users of the code is the attribution.

The license is short and readable. https://github.com/marcobambini/gravity/blob/master/LICENSE

You're right, the github license page contains the explanation "Licensed works, modifications, and larger works may be distributed under different terms" so changing the license is possible. So what remains is attribution. The Parrot author could add a single sentence "based on X" somewhere. One could question the ethical aspects but it would probably fulfill the attribution requirement.

Not quite sufficient, but close. From the license:

"...subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software."

One could question the ethical aspects

Something like taking an MIT-based project and just rebranding it - or basically just rebranding it - sure. It does put a bad taste in one's mouth. And that does seem to be what the other project did. (...So far. Maybe they intend to go a different way with the language, and we just happen to see it at this primordial state because it's being developed in public. Who knows.)