Preview is available the application's apk GMail for Android 4.2 Key Lime Pie: not only are we can see the news, but now we can also install it on our Android smartphone
This guide is continuation from the previous guide : install app gmail 4.2 on android phone

• Root Permissions
• Apk GMail (download)
• A smartphone with Android 4.0 or higher

Guide

• Download Gmail 4.2 and rename it to Gmail2.apk
• Make a Nandroid backup to safety, much later cleared any Gmail version
  (you can use Titanium Backup or ROM Toolbox to delete the old version of GMail, but if that doesn't work, follow the information below)
• Delete GMail versions present in/system/app and in/data/app. The recognize thanks to the name "Gmail.apk," or "com.google.android.gm-[number] .APK," and the typical icon
• You may need to delete also the fil. equal that odex were found along with the apk described above, but much depends on the type of drive you have installed
• Reboot and install Gmail 4.2
• If you cannot install the APK, try using the command ADB adb install Gmail2.apk

That's all ... the procedure is very simple, so don't worry if the first shot fails, repeat everything calmly. Remember also that the apk installed in this way will not receive official updates from Play Store.