I came on here to complain that wmaker isn't on this list. IMO it's the best WM for X.
As for the rest of GNUstep, at various points during the 2000s decade I'd been interested in GNUstep, developed a little bit with it, used to use apps like GNUMail, etc. And it's definitely a cool project. But I have to say it's by no means a lightweight toolkit. It seemed pretty sluggish for the entirety of the time I used it. Maybe it's fast now that hardware is ridiculous. But I think if WindowMaker actually used GNUstep's AppKit (it does not), it would be a lot more sluggish than it is, and that would make it pretty bad. I've used wmaker since the 90s and it's never felt like a bottleneck. Can't say the same about the rest of GNUstep.
As for the rest of GNUstep, at various points during the 2000s decade I'd been interested in GNUstep, developed a little bit with it, used to use apps like GNUMail, etc. And it's definitely a cool project. But I have to say it's by no means a lightweight toolkit. It seemed pretty sluggish for the entirety of the time I used it. Maybe it's fast now that hardware is ridiculous. But I think if WindowMaker actually used GNUstep's AppKit (it does not), it would be a lot more sluggish than it is, and that would make it pretty bad. I've used wmaker since the 90s and it's never felt like a bottleneck. Can't say the same about the rest of GNUstep.