Someone who thinks that OpenStep is a good way to build end-user applications, I imagine. I've never used it myself, but I know that it had an excellent reputation once upon a time. I wouldn't be surprised if it's better than Qt or gtk+.
> Working on top of the X11 implementation also doesn't really provoke trust to look into the project.
X11 is the standard display technology for Unix and Unix-like systems. Not working atop X11 would provoke distrust. I imagine that you are actually advocating for Wayland. Wayland may actually be the future, although I am still unconvinced that its design is sound (I worry that it throws out the baby with the bathwater).
edit: I did not realise that there is a Wayland backend for GNUStep.