I hate to repeat myself but I must say thanks again to all of you guys (catymag, Neil, kunix and...). I'm really touched with your willingness to help.

@Neil
It's not excuse but, after 10 hours of work + preparing for exam, my brain just doesn't work properly. Yes, you're right, I completely forgot that I'd unsuccessfully tried to update software dragging nuvi23xx_GCDFile__370.gcd into webupdater. Before that there wasn't any gupdate.gcd. I agree with you, it looks like boot.bin is an inevitable option. I'll take a chance with it this weekend and let you know whether everything went ok or not (PM of course).