In a document of this type, I would not even try to use a gutter. Just
increase the size of your left margin (or the outside margin, using "Mirror
margins," if you're duplexing). In the landscape sections, set the left and
right margins to what your top and bottom margins are in the portrait
sections and increase the top margin to create a "gutter."

If you're duplexing, you'll need to use "Different odd and even" headers and
footers, keep the margin the same, and add space to the header on odd pages
and the footer on even pages using Space Before/After.