What you are experiancing is known as a boot loop, but if you can get the phone into download mode it is recoverable.
If you want stock 2.1 it will need reflashing with KDZ updater
First you will need KDZ updater and the right rom for your country, the roms can be found here
Stock 2.1 and KDZ updater can be found in this post
http://androidforums.com/optimus-gt...nogenmod-7-android-2-3-4-avaliable-gt540.html
It doesnt matter which version of KDZ you use.
Extract the files from the KDZ zip you downloaded to a folder called Android, in the root of the C: drive, so go to my computer, C: then create a new folder called Android.
Also save your 2.1 rom into there, ignore the rom that comes with the KDZ unless you want to flash a custom rom.
Also install the msxml.msi file and the LGAndroidDriver_WHQL_ML_Ver1.0_All_Win7.exe file
Connect the phone to your PC and and hold the volume down button while you switch it on, you should get a black screen displaying "Download Mode".
Then on your PC open the KDZ_FW_UPD program and accept any UAC popups and under "Type" select "3GQTC" and Phonemode select CS_Emergancy then go to KDZ File and select the KDZ file you downloaded and hit "launch software update"
DO NOT TOUCH THE PHONE OR USE YOUR PC UNTIL THE UPDATE IS FINISHED AND BY NO MEANS INTERUPT THE INSTALL OR YOU COULD PERMINANTLY BRICK YOUR PHONE.
Sorry about the caps, just wanted to emphisize that part.
This should then flash the clean 2.1 rom to your phone.
I havent used the stock bootloader for awhile so i'm not 100% sure that the volume down while you power on will get it into download mode, I just know that that is how I would do it.