thanks for the question
Gedit is a thing of beauty and thanks to its plugin architecture it's phenomenally powerful but outside a handful of developers, most people don't realise it. That side panel, doesn't haven't to be a flat list, and in fact I'm experimenting with turning it into a tree view 'binder'. I'm currently writing quite a few plugins, some of which provide some writing-centric features and others to automate my fantasy world building procedures. The first plugin I wrote, compiles to HTML, ePUB and PDF - adding mobi format support would be trivial. It's astonishing what you can do with Python and Gtk. I was planning on sharing the code once I've finished or even packaging it up in a PPA.
As for plain text, for fiction writing, I use Markdown. Gedit understands markdown so it displays bold and italic in styled format thanks to Gtksourceview support.
I haven't made a song and dance about this because:
1. it's not meant to be a Scrivener competitor - think of it as an advanced Markdown/AsciiDoc mode optimised for writers
2. plain text is not for everyone
3. Linux is not for everyone
4. Quite a bit of the functionality I'm creating is more relevant to my day job (technical writer) where I use AsciiDoc on Linux.
Mostly though, I'm waiting for the iOS version to appear. If that does everything I need then including seamless syncing, I may be tempted to just use that and stick with Rich Text for my fiction projects as much as I loathe it. I have one very big project in Scrivener (since 2007) and I'm loathe to move it, everything I've written since 2010 has been in plain text.