For grouping related items in a dialog, I'm want to use a HIG style GtkFrame:

GtkFrame (bold label, no visible border)
    GtkAlignment (12px padding left, 6px padding top)
       <child widget>

But the label of the GtkFrame is indented a few pixels. Since the frame has no borders, this indentation appears as extra left padding between the frame and its parent widget. Also the 12px padding of the GtkAlignment seems to be reduced by this indentation.

Currently, the only solution is not using a GtkFrame:

GtkVBox
    GtkLabel (bold label)
    GtkAlignment (12px padding left, 6px padding top)
       <child widget>

I find it rather strange that it's nearly impossible to obtain the recommended style [1,2] with the widget that is designed for this type of layout. I think the indentation should be removed when there is no visible border (e.g. GTK_SHADOW_NONE). Bug 96374 (comment 7 and 12) mentions the (removal of the) indentation, but as far as I can tell it was never implemented.

[1] http://developer.gnome.org/projects/gup/hig/2.0/controls-frames.html
[2] http://gnomejournal.org/article/44/three-simple-tips-for-interface-design-you-should-know