Gets a `GtkWidget` that has previously been set as gutter.
See [[email protected]_gutter].
win must be one of gtk_text_window_left, gtk_text_window_right
, gtk_text_window_top, or gtk_text_window_bottom.
| this |
a `GtkTextView` |
| win |
a `GtkTextWindowType` |
|
a `GtkWidget` |