Thank you Poshu for pointing out that the flag #PB_Ascii has to be added for compilation in Unicode mode and
thank you Remi for pointing out that the *FontDescription string has to be freed. I totally overlooked that...

I have updated my first code example:
Code:
EnableExplicit
ImportC ""
pango_context_get_font_description(*PangoContext)
pango_font_description_to_string(*PangoFontDescription)
EndImport
Procedure GetGadgetFontInfos(GadgetID.I)
Protected *FontDescription
Protected PangoContext.I
Protected PangoFontDescription.I
PangoContext = gtk_widget_get_pango_context_(GadgetID(GadgetID))
If PangoContext
PangoFontDescription = pango_context_get_font_description(PangoContext)
If PangoFontDescription
*FontDescription = pango_font_description_to_string(PangoFontDescription)
If *FontDescription
MessageRequester("Info", PeekS(*FontDescription, -1, #PB_Ascii))
g_free_(*FontDescription)
EndIf
EndIf
EndIf
EndProcedure
OpenWindow(0, 270, 100, 170, 45, "Font infos")
ButtonGadget(0, 10, 10, WindowWidth(0) - 20, 25, "Get font infos")
Repeat
Select WaitWindowEvent()
Case #PB_Event_CloseWindow
Break
Case #PB_Event_Gadget
If EventGadget() = 0 And EventType() = #PB_EventType_LeftClick
GetGadgetFontInfos(0)
EndIf
EndSelect
ForEver