The problem with free UNIX clones is that everyone does its own thing, as such you don't have a nice SDK that the community at large can enjoy and effort get scattered all over the place.

Picking Gnome as example, not only do you have what you have mentions, but you also have Anjuta and Builder.

Although Ubuntu now kind of has an SDK based on top of Qt/C++.

However none of them close to the developer experience that Delphi and C++ Builder used to have. Or nowadays the .NET and UWP stack have.