If the window uses GadTools menus and GadTools gadgets, then use the
GT_GetIMsg() and GT_ReplyIMsg() functions described below (or
GT_FilterIMsg() and GT_PostFilterIMsg(), if applicable).  However, if the
window has GadTools menus, but no GadTools gadgets, it is acceptable to
use GetMsg() and ReplyMsg() in the usual manner.

Additionally, no context need be created with CreateContext() if no
GadTools gadgets are used.  For more about these functions, see the
section on "Other GadTools Functions" later in this chapter.