According to the Intuition Reference manual from Commodore, chapter 4 page 51:
The unique feature of a Gimmezerozero window is that there are
actually two “planes” to the window: a larger, outer plane in which
the window title, gadgets, and border are placed; and a smaller, inner
plane (also called the inner window) in which you can draw freely
without worrying about the window border and its contents. The top
left coordinates of the inner window are always (0,0), regardless of
the size or contents of the outer window; thus the name
The area in which you can draw is formally defined as the area within
the variables BorderLeft, BorderTop, BorderRight, and BorderBottom.
These variables are computed by Intuition when the window is opened.
To draw in normal windows with the graphics primitives (for instance
to draw a line from the top left to somewhere else in the window), you
have to start the line away from the window title bar and borders.
Otherwise, you risk drawing the line over the title bar and any
gadgets that may be in the borders. In a Gimmezerozero window, you can
just draw a line from (0,0) to some other point in the window without
worrying about the window borders.
The Gimmezerozero window uses more RAM than other window types and
degrades performance in the moving and sizing of windows. There can
be a noticeable performance lag, especially when several Gimmezerozero
windows are open at the same time.
There are some special variables in the Window structure that pertain
only to Gimmezerozero windows. The GZZMouseX and GZZMouseY variables
can be examined to discover the position of the mouse relative to
the inner window. The GZZWidth and GZZHeight variables can be used to
discover the width and height of the inner window.
Here is a link to the book. The full definition of the Window stuct is shown on appendix pages B20-B21 (and corroborates Brian H's snippet of the header file).