Here's how to add a Gallery like the one seen on this site.

My site has done it by using a forum called "Gallery" that is set not to display in the forum listings (set its display order to "0" in the channel manager).
Create a link in the navbar to that forum and call it "Gallery".

Then, go into that forum, and using Site Builder, set the tabs to display "Latest Activity" by default, and not to display "Topics" whatsoever. Make this change by specifying a new template as you save, otherwise it will affect all forums.

Finally, a little bit of css to add to the template css_additional.css:
/* Album thumbnails fit in a post */
.b-gallery-thumbnail-list__item {
    display: inline;
/* End Album thumbnails fit in a post */
It's not perfect but it's quite handy!