What i found out so far is that resizing is different between GTK2, GTK3 and Windows. Try this tiny program and check the label border on different systems.
Code:
OpenWindow(0, 0, 0, 200, 200, "Test", #PB_Window_SystemMenu | #PB_Window_Normal | #PB_Window_ScreenCentered | #PB_Window_SizeGadget)
TextGadget(0, 0, 0, 100, 32, "This is a test string with or without line break", #PB_Text_Border)
Repeat
Until WaitWindowEvent() = #PB_Event_CloseWindow
Here is how it looks with GTK2:

The label wraps the text but the gadget will
not be resized.
Here is how it looks with GTK3:

The label wraps text as well, but
resizes the label.
My guess for Windows is, that the label does not wrap by default, but the gadget will be auto-resized in width. Maybe someone can confirm this.
My proposal is to make the behaviour on Linux the same like on Windows. Maybe PB can call the
Code:
gtk_label_set_line_wrap_(gadget, false)
immediatly after creating a new Label gadget.