It appears Google accidentally leaked a redesigned version of Gmail for Android at Google I/O last week.
Android Police first noticed the redesign during the "Structure in Android App Development" presentation by the Android Design Team, when a slide clearly revealed the Gmail for Android app with a left-side navigation drawer. The "action bar" along the bottom also seems to be missing.
The current version of Gmail for Android has the usual pull-down menu, but the iteration depicted in the slide notably features a drawer that pulls from the left with the inbox, as well as labels, starred items, and other Gmail tools. The video below shows off the new look at about 23 minutes.
While these changes might roll out sometime in the near future, it is also possible that the redesign is just a concept or mockup. Only time will tell.