Actually it is quite possible.
I used the google app package from Cyanogen (cyanogenmod.com). Look for the package "gapps-mdpi-tegra-20101020-signed.zip". Copy that on SD card and use recovery mode to install (no need to wipe).
There is one catch: there is not too much space on /system partition and you need to delete some of the apps from that package before attempting to upgrade. Also you need to edit "updater-script" and remove those apk. Even more, you may need to re-mount /system in Read/Write mode and delete some from /system/app.
It's not too hard, but like Strogino said, it's not straigtforward - you need a little bit of experience...