Prior to this update, swiping left or right on an email in your inbox would archive it. This is a handy tool, but the limitation to just archiving has always been slightly irritating. In v8.5.20, jumping into the Gmail app and going to Settings -> General Settings -> Swipe Actions now allows you to customize the action for both left and right swipes.
The Swipe Actions page shows what you have each direction currently set to and tapping the blue Change button near the top right of either one allows you to set it to one of the following:
Mark as read/unread
This is something that really should have been in Gmail all along, but we're still thrilled to finally see it make an appearance.