Is there any way to get gestures (e.g. trackpad- or Magic-Mouse-swiping left/right) to work for navigating forward and backward in the Mathematica help browser, particularly on a Mac? If not, what are the keyboard shortcuts to navigate forward and backward?
According to the help page
ref/menuitem/DocumentationCenter, the keyboard shortcuts to navigate one page backward or forward in Mathematica for OS X are ⌘ [ and ⌘ ]
(or alternatively ⌥ ⌘ ← and ⌥ ⌘ → ), so you could use a third-party app like for example MagicPrefs to bind those keys to a trackpad gesture.
For the sake of completeness, the corresponding shortcuts on Windows & Linux systems are Alt ← and Alt → respectively.