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?