Thanx to HwGuru and Rambo (two guys of nas-2000.org) who created an ad-hoc version of their Flashtool, some days ago i got my unbricked GigaNAS working again using the JTAG method!
I simply soldered a 20pin connector to the PCB, turned on the NAS by shorting pin 1 and 17, and i wrote the NAS bootsector using last FlashTool release.
(be aware: there's still a bug regarding the "JTAG test" button that cannot be used twice; to do so simply restart the software)
Anyway, now you dont have to be worried if you erased the bootloader because it could be restored back, because there's a definitive way to unbrick it!
I'll update NAS unbricking guide accordlying.
As always, if you cannot do it by yourself ask help.