Tablet listed under adb devices, but not under lsusb while in recovery
I'm afraid the stock recovery (as pointed out in the comments) doesn't come with ADB enabled, which is why your device is neither shown with adb devices nor with lsusb when you've booted it into &lt;3e&gt;. The only way around this (if you really need to access the tablet via ADB while booted into recovery-mode) is flashing it with a custom recovery, such as clockworkmod-recovery or twrp – which again requires your bootloader to be unlocked (see: bootloader-unlock).

Note that the recovery image must match your device, so you cannot just pick any. I'm not sure whether there's any custom recovery available for the GEM1000; good points to check that out is looking at XDA-Developers and the websites of the corresponding recoveries.