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