set_gutter_size() can currently be used to set the width or height in pixels of the gutter where the dividing handle is based, only where the 1.2.* series of GTK+ is being used to compile PHP-GTK. It does nothing in the win32 version.
Please note that this function is more than deprecated in GTK2; the gutter_size property does not exist in later releases. Do not use this function.