Some good news - the G Pad got unbricked thanks to XDA member Mixinitup4Christ (http://forum.xda-developers.com/showpost.php?p=48617560&postcount=15) who provided the stock recovery image which did whatever TWRP couldn't do and finally allowed the tablet to boot.

My earlier troubles with bootloader unlocking and flashing recovery are here: https://plus.google.com/u/0/107797272029781254158/posts/85v5cX8JMQQ.

The lesson is: ALWAYS REBOOT AFTER fastboot oem unlock BEFORE FLASHING CUSTOM RECOVERIES OR ELSE....

Hopefully, this will help others flashing and unlocking things in the future. 
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