First, a small bug on this bug submission page: I'm reporting an issue with Writer 7.2.1.2 but there is no option to select that version from the list. Nearest options are 7.2.1.1 rc and 7.3.0.0 alpha
Note that I downloaded from https://www.libreoffice.org/donate/dl/deb-x86_64/7.2.1/en-US/LibreOffice_7.2.1_Linux_x86-64_deb.tar.gz a few minutes ago.

Version: 7.2.1.2 / LibreOffice Community
Build ID: 87b77fad49947c1441b67c559c339af8f3517e22
CPU threads: 8; OS: Linux 5.11; UI render: default; VCL: gtk3
Locale: en-AU (en_AU.UTF-8); UI: en-US
Calc: threaded

The release notes for this version described a new feature:

 Page styles can now have a gutter margin blog post (Miklos Vajna, Collabora)

The UI is confusing because in the context of a book or magazine, the gutter refers to the binding, which normally means the centre (or I suppose the top edge if you were producing a calendar or similar - though maybe it could be the bottom too, in that case, or even alternating top-bottom).
Yet the UI only provides options of Top or Left.
For books, the gutter is normally in the centre: so, on the right for even pages and on the left for odd pages.

I read the blog post https://vmiklos.hu/blog/sw-gutter-margin.html but that seemed to define gutter as something I'm not familiar with - a feature related to a program called Collabra - rather than to the term as more commonly used I think. I couldn't match it up with the term I'm familiar with.

See e.g. 
https://www.abaa.org/glossary/entry/gutter or
https://www.printivity.com/insights/2019/05/18/how-to-design-for-gutter-margins/ or any number of other sites, such as https://en.wikipedia.org/wiki/Bookbinding

So the new term and feature has confused me.