I have the APK File to the pre-release of Gmail Material Design. I went to Google Play Store and uninstalled my current, up-to-date Gmail package. When I install the material design APK, it tells me that an app with the same signature already exists. What app? I just uninstalled it, it should just install, no? After all, I even went to ES File Explorer >> App >> Gmail >> Uninstall. I am not rooted and I use Blu Studio 6.0 HD.
NOTE: The Gmail 5.0 apk is currently causing a lot of issues and force closes on numerous devices, and without root I doubt this will work. Proceed at your own risk. As a non-root user I would wait until Google roll out the update.
The signature already existing means that you already have something with the same package installed: Gmail! Gmail may well be a system app on your device and is not being unistalled properly. The actual /system/gmail.apk may remain despite it seemingly uninstalling, especially on a non-rooted device. The conflict is happening as you are trying to install an app you already have.
The easiest way around this is to uninstall Gmail. root users can simply uninstall the application, and install Gmail 5.0. You could disable your existing Gmail application
Settings > Applications > Gmail > Disable and try to install the apk, but as I said at the start there is a very high chance you will get endless force closes.
One of the workarounds on XDA for ROOT ONLY:
I am showing this less as a guide, and more as an illustration of the issues at the moment.
Rename old gmail.apk Pushed/renamed new Gmail.apk Change permissions
Then clicked on the app in Root Explorer and installed it --- this time it installs successfully.
Then I deleted all app cache and app data.
Changed the r.build.type=userdebug