And those are? The GPL is far more restrictive, I can't see anything valuable that the GPL would contribute here.

As he says in the linked tweet: the major value the GPL contributed is the code being open-sourced in the first place, since his co-copyright-holders would never have agreed to license it BSD. The alternative to the GPL, in this case, was "(c) All Rights Reserved", which is still how most other old games are licensed.

So valuing that his co-holders didn't want to opensource is good for the open source community? That seems backwards.

Que? They were obviously fine with open-sourcing, just not under BSD license.

GPL is more restrictive for the developers, but more liberating for the users. If you care about your users, you'd use GPL.

It's just a matter of perspective.

GPL lets you release source code and enforce that no one can modify it without releasing their modifications. A lot of people like that "restriction" because it maximizes the amount of source people have access to.