I'm working on a Gtk2 application that basically solves the same problem. It adds a sidebar with things like a file browser to a GVim editor. For getting a feel for a foreign codebase I find it much easier to use something like this than to switch between navigation/file view all the time.

I tried plugins like NERDTree, which are awesome in one way because they work everywhere vim works (like in the terminal). However, I didn't really like their feel in combination with GVim and tabs.