The gutter margins need to be sufficient so that the spine or the readers eyes are not strained while reading.
How to set createspace gutter.
The gutter is in addition to the inside margin.
If you go to the margins tab of the page setup dialog and select bookfold or mirror margins from the multiple pages drop down and then try increasing the gutter and then the inside margin you will see what affect each one has.
Review your project setup.
These both mean the same thing so you only need to set one.
So if you want the inside margin to be 0 5 for all pages add this value in the gutter margin box and leave the inside margin field blank.
If you re using mirror pages the value for the inside margins will be set in the gutter field.
The spine of the book is calculated based on the number of pages and on the type of paper you plan to use.
Page extent minimum inside margin inches minimum inside margin mm 24pp 150pp.
The minimum size of the gutter varies according to how many pages there are in your book.
Lastly set up the spine.
If everything looks okay submit your files for review.
However traditionally gutter spines have also been the smallest of the margins since the recto and verso pages laid together each with a large gutter spine will create an unpleasingly blank look to the center of the book.
You can go back and make changes if you need to.