Ok, guys. Start your engines - here's the download and instructions you need to get the leaked Gmail 4.2 up and running on your device. Keep in mind that this is an unreleased version so some things may be buggy or broken, though I haven't run into any problems.
If you somehow don't know what I'm talking about, we have the next, unreleased version of Gmail, and we're finally allowed to share it with you. Gmail 4.2 finally has pinch-to-zoom, a nifty new swipe-to-archive/delete feature, and a bunch of other things. Just for fun I'll embed the video again:
Note: This APK should work on Android 4.0+. You do need to be rooted.
Update 10/15/12 6:21PM: Google asked us to take down the mirrors, and we have complied with their request.
Android Police mirror #1 Android Police mirror #2 Android Police mirror #3 Android Police multi-mirror
The Gmail 4.2 APK is signed by Google, but with a different key than the one in the Play Store (we checked the signatures - they differ). One is probably the test key and the other the production key. For this reason, you can't simply install it as an update and therefore need to be rooted with the ability to mount the /system directory for writing in order to remove the existing Gmail APK.
Important: Once Google updates Gmail in the Play Store, you will want to revert this process or at the very least delete the Gmail 4.2 APK you installed here if you want to update it from the Play Store.
OK, let's get down to it. You'll probably need ADB installed on your computer (here's an easy way that doesn't require downloading the Android SDK). There is a chance that this might work without ADB by just removing the APKs mentioned below and installing the new one, but it didn't seem to work reliably for everyone. This should, though:
Download Gmail 4.2 from one of the mirrors above.
Rename it to Gmail2.apk.
Make a Nandroid backup in case something goes wrong (this is always a good thing to do before messing with your device).
Remount /system for writing (via your favorite root-aware file explorer).
Delete (or move if you want to save the APKs) all other versions of Gmail.
Update: We've seen multiple successful reports of simply uninstalling Gmail using Titanium Backup or ROM Toolbox instead of performing this and the next step. If that works for you, go directly to step 7. If not, proceed below.
Usually there are 2 or 3 copies. One will be in /system/app (that's the original Gmail that was pre-installed on the device) and there may be a few in /data/app (these are the updates installed from the Play Store). They'll be called "Gmail.apk," or "com.google.android.gm-[number].apk," An easy way to tell is to look for the Gmail icon.
You may need to delete the .odex files matching the APKs in step 5 too, if you have them. Neither I nor Artem had to do this, but there are many different ROMs out there, and we're trying to cover all bases. Ignore this step and only come back to it if something doesn't work later.
Reboot. Once you do, you should have no Gmail as far as Android is concerned.
You can try to install the Gmail 4.2 APK you downloaded at this point by tapping it in your favorite file explorer (move the APK to your device first, of course). If that doesn't work, or if you prefer ADB, fire up a command line and type adb install Gmail2.apk.
If you get an error message, you probably didn't delete all copies of Gmail. If you don't feel like playing with ADB for Step 8, I've also gotten it to work by just copying the APK to /data/app, but that's not as fool proof as "ADB install".
Enjoy and report your results in the comments!