That video above shows a small preview of whats new in Gmail 4.2.
1. Download the apk from HERE.
2. Make a titanium backup of your Gmail App first.
3. If you are have your Gmail app installed in Data/App, Delete them first, if not skip this step. You won’t lose your Gmail data, dont worry
4. Copy the Gmail.apk using any root explorer to System/App.
6. Launch the Gmail app, and your Gmail data will be still there.
I have yet to try it myself, so please do this at your OWN caution. Let me know if it does or doesnt work for you,
Credit: XDA forum