How do I right (or otherwise) justify a GtkLabel?

Are you sure you want to justify the labels? The label class contains the gtk_label_set_justify() function that is used to control the justification of a multi-line label.

What you probably want is to set the alignment of the label, ie right align it, center it or left align it. If you want to do this, you should use:

void gtk_misc_set_alignment (GtkMisc *misc,
                             gfloat xalign,
                             gfloat yalign);

where the xalign and yalign values are floats in [0.00;1.00].

GtkWidget       *label;

/* horizontal : left align, vertical : top */
gtk_misc_set_alignment(GTK_MISC(label), 0.0f, 0.0f);

/* horizontal : centered, vertical : centered */
gtk_misc_set_alignment(GTK_MISC(label), 0.5f, 0.5f);

/* horizontal : right align, vertical : bottom */
gtk_misc_set_alignment(GTK_MISC(label), 1.0f, 1.0f);