Some progress with the GtkGrid. After trying to get my expose method called without success I realized I have to add the GDK_EXPOSURE_MASK to the main gdk window of the widget.

Then I played with some gtk_paint_* functions and this is what I got. It starts looking like a real Grid :)

This is my tentative Roadmap for the Grid:

1. Create a GtkGridColumn class that will hold the renderer for that column
2. Add GtkGridColumns to the GtkGrid and modify the expose event to use the renderers instead of hardcoded drawing stuff.
3. Add a model property to the GtkGrid and make sure it is a GtkListStore
4. Work on some key events to navigate through the grid
5. Add scrolling support
6. Add more gdk windows for the header and the left sidebar
7. Make the header and sidebar functional by letting to select whole rows and columns
8. Gettext support for properties
9. Drag and drop of columns and rows

You don't see dates on that roadmap, that's right because I don't know how long will it takes me to write each step as this is the first widget I write.