FontChooserT.setFontMap

Sets a custom font map to use for this font chooser widget.

A custom font map can be used to present application-specific fonts instead of or in addition to the normal system fonts.

FcConfig *config;
PangoFontMap *fontmap;

config = FcInitLoadConfigAndFonts ();
FcConfigAppFontAddFile (config, my_app_font_file);

fontmap = pango_cairo_font_map_new_for_font_type (CAIRO_FONT_TYPE_FT);
pango_fc_font_map_set_config (PANGO_FC_FONT_MAP (fontmap), config);

gtk_font_chooser_set_font_map (font_chooser, fontmap);

Note that other GTK widgets will only be able to use the application-specific font if it is present in the font map they use:

context = gtk_widget_get_pango_context (label);
pango_context_set_font_map (context, fontmap);