bengorro: Did you check the MD5 sum of the download with the MD5 sum at the download site? This is used to determine if a download is/became corrupt. By checking to see it the sum is the same on both ends you will know if the download is good or corrupt.
This is what I suspect has happened to you.
It might be because you did not have root access to your phone? I'm doubtful of this, but you never know.
I also noticed it was to go from 2.1 to 2.3. You had 1.6 and might have needed to upgrade to 2.1 first?
I will need to look over the actual Tutorial and links (even download them and check the MD5 sum), then maybe I can see what happened, and how, if anything, can be done.
Can You get your phone into emergency mode?
EDIT: It looks like you should have gone here first:
http://androidforums.com/optimus-gt...-1-android-2-2-here-gt540-16.html#post2094180 Then you would have been able to get into fastboot screen.
Well.....................lets see if there is anything we can do to save the phone. Getting the phone into emergency mode will be one of the keys (Make sure PC is on with the USB cable plugged into PC only,pull the battery and hold power button for 30 sec. then re-install battery. Hold both the Volume up and down buttons then press the power button and connect the USB cable to the phone, this is tricky and might take a few tries). The another key is to get your PC to recognize the phone in emergency mode, which I will explain once you let me know if you can get it into emergency mode.
Also check and see if it will go into download mode(doubtful). With the power on, hold the volume down button until you see download mode displayed (you shouldn't need to hold it down more than a minute)