I take no responsibility for any harm that may come to your phone trying this.
You must be rooted for this to work
check out the links at the bottom on this post for ninjamorph stuff
****The new image must be the same size as the old image for this to work****
I was able to get the notification changed using ninjamorph and rootexplorer.
1. rename your image stat_notify_email.png
2. Open ninjamorph and start a new project.
3. go to data/app/com.google.android.gm-1.apk
4. When you click it, it should extract the folder.
5. On the next screen click copy it(lower right corner).
It should tell you where the copy was saved. For me it was on my phone storage in androidthemes/workspace
From there i went to rootexplorer( you could also do this part on your computer).
1. open androidthemes/workspace/com.google.android.gm-1.apk/res
2. Locate these file:
3. This may just be an extra step but i went through each of these files and added .bak to the end of each original image so it looked like this: stat_notify_email.png.bak (this will keep the original as a backup in the folder).
4. Copy/paste the new image named stat_notify_email.png into each of these folders.
If you don't rename the files when you paste you will just replace the original.
5. Once you have placed the image in all the file you will go back to ninjamorph and click finish project.
6. Click the com.google.android.gm-1.apk
7. It will then create package
8. It will ask if you are sure you want to replace the .apk....click yes
9. It will ask if you want to optimize using zipalign...click yes
10. It will ask if you want to clean up/delete project...Click yes or no(up to you)
11. Once it is done, reboot phone and it should have the new notification icon but it will have a dark tint from the notification bar.
I have not done that much with ninjamorph so there probably is a shorter way to do all this.
Here are a couple links to using ninjamorph:
Hopefully this works for you.