The gutter position box is not available and determined automatically when you use the mirror margins 2 pages per sheet or book fold option.
How to set a left gutter margin.
If you enable mirror margins with a top gutter setting it will be changed to left making it useless for duplex documents.
Gutter margins are adjustments made to horizontal positioning in addition to the standard left margin setting.
But in addition to that margin you also want a gutter.
This accounts for the binding area.
This feature is not available in word online.
In such a case type a number in gutter that will allow enough room for the binding and use the drop down to indicate whether the binding will.
This margin setting will be used in each new document you create based on that template.
Change the size for the top bottom left and right margins.
Select layout margins custom margins.
The left gutter setting can be combined with mirror margins as shown below.
The purpose is to modify the horizontal margin to allow for the binding of documents within a folder.
First if you intend to have left and right hand pages you have to tell word that.
In the gutter position box click left or top.
Word typically allows you to choose whether to position the gutter margin of your document at the top or left of the page.
The adjustment is added to the left margin for the front face but subtracted from the left margin for the rear face of duplex pages.
Select set as default to set the new margin settings as the default for the current template.
In the gutter box enter a width for the gutter margin.
The gutter goes on the left side of right hand pages and on the right side of left hand pages.
Type numbers indicating the width of your margins in the top bottom left and right fields.
In effect a 1 inside margin combined with a 1 gutter is the same as a 2 inside margin.