You don't have to be rooted to install Gmail 5.0 using the Android Police (or any other non-Playstore) APK. You do, however, need to set your phone to allow installs from sources other than the Play Store.
I have my Nexus 7 2012 set to allow external installs, and Gmail 5.0 looks and works great on that.
I prefer to keep the external installs blocked on my Moto X as I also use Touchdown for my corporate email on that, so I'm running 4.9 on that (but I'm also an Inbox user and that gives me all the material design view of my gmail I care to have - LOL).