Actually, GNUstep has always had an Interface Builder:
https://www.gnustep.org/experience/Gorm.html
My own take is actually that GNUstep spent too long trying to be an OpenStep successor instead of being a way to run Mac apps on Linux.
It took them ages to even clarify if it was a desktop environment or an SDK (and I am not sure it is even clear now).
There has never been a tonne of love for Objective C either. Pretty much the only reason to use it for most people has been because you had to for access to Apple APIs. Which would be the only reason to use it on Linux too.
It always amazed me that Darwin + GNUstep did not result in a macOS clone. Neither of them really went anywhere.
Have you seen Gershwin?
>It took them ages to even clarify if it was a desktop environment or an SDK
Yes. In my eyes it’s both. I mean that in a good way, relative to the way Oberon is both an OS and a language or that Smalltalk is both a language and its authoring environment. Or how Bash is an interactive command shell and a language.
Some people might not like that tight coupling, but sometimes it can be really powerful.
I think there was a funding campaign once where a developer offered to re-implement the missing Apple APIs, but since they basically put up a standard salary it didn't reach the funding target.
> It took them ages to even clarify if it was a desktop environment or an SDK (and I am not sure it is even clear now).
Yes they still have no idea what they are trying to do.