It would be great if the KiCad’s gerber viewer would be a full-blown editor, but unfortunately the development resources are limited and the wishes are practically impossible. All the work must go to the actual design editors. A gerber editor would be a totally new project, and not a small one.
I don’t know if the current gerber viewer implementation would be a good starting point for an editor. Editing gerbers is so much different than viewing them. The first obstacle is that there’s no standard representation format for the features – I mean changeless reading → writing cycle. I once tried some editor and it totally restructured and rewrote the file. An editor would probably need a completely new engine first.