I've just installed the latest official Gingerbread 2.3.3 (V20E) on my daughters P500
I've downloaded the 'Open European' firmware from here:
http://csmg.lgmobile.com:9002/swdata/WEBSW/LGP500/ANEUBK/V20e_00/V20E_00.kdz
And used a KDZ Firmware Updater
Download it from here:
KDZ_FW_UPD_EN.zip
I followed a procedure similar to this:
How to downgrade your firmware (Lg Optimus one p500) Lg Optimus One P500
For me it worked without a hitch. It was an unbranded phone before upgrade.
P.S. The AVG seems to give a false positive on KDZ Firmware Updater - but can't confirm that 100%