Yes they absolutely can? Anyone can. I can.

The thing the GPL requires is that I also provide it for free. Now, why would anyone buy a free thing? To support the devs. To encourage this sort of business model. To get a build that's known to be working and supported and not have to deal with the hassle of compiling things themselves.

I'm pretty sure you can. But if it's GPL, you need to provide source to the buyer, and the buyer can publish that source for free.

Yes they can. Firstly, you can do anything and then someone may or may not sue you for it and they may or may not win. Secondly, if you're the copyright owner you can do whatever you like.

I think their plan is to make assets closed licensed. It's all pretty lame. Now any open source code contribution is packaged up and sold with someone else getting the check.

The owner of the copyright can do as they please with the licensing. What I've not seen is someone retroactively changing licensing terms, but even that might be possible.

The blurb in game suggests the multiplayer side of things will be open, the 'premium' thing on steam gives the one closed bit, which is the single player campaign.

(In theory!)

What do you mean?