Not easy, but you can do it yourself.
* your phone needs to be rooted.
* Android SDK or simply ADB installed
Let’s do it:
1 - Install Apk Manager (found here)
2 - 'adb pull /data/app/com.google.android.gm*.apk' (get your Gmail updated version from your phone)
3 - copy this APK file in Apk Manager 'place-apk-here-for-modding' folder
4 - launch Apk Manager
5 - choose 22, pick your project
6 - choose 9 to decompile
7 - enter in projects folder, edit the 'AndroidManifest.xml'
8 - on every lines you find android:protectionLevel="signature" change them to android:protectionLevel="normal", save your file
9 - in apk manager, choose 11 to compile the APK
10 - answer yes to 'system app'
11 - answer yes to 'copy original files'
12 - go in apk manager, 'keep' folder, delete 'AndroidManifest.xml' and 'resources.arsc'
13 - go back to apk manager script, press enter
14 - in apk manager, choose 12 to sign the APK
15 - pick the final APK file in 'place-apk-here-for-modding' folder
You’ve got it ! You have your own Gmail modded application.
Now, you need to install it … check here for the manual
Nota: sorry, but I won’t help you on this. If you have questions, go on the XDA forum.