The update is only available through the LG Mobile Support tool so you will need to download that and use a computer to get it. Considering it's 178MB, it’s hardly too great a surprise that you cannot get it over the air.
So once you apply the update, your T-Mobile G2x Android version will be brought up to Android 2.3.3, which is the last major release of the smart phone branch of the Google OS. There's a minor 2.3.4 update after that, but presumably it will be delivered to the G2x customers at a later stage.
What surprises us a bit is that LG chose to update the G2x before the Optimist 2X. The international version of the handset has been around for longer and it certainly should have sold more units, but alas it's still stuck with Froyo.
This might have something to do with the class action lawsuit that the G2x triggered. If the update brings a fix to those issues plaguing the device since launch, LG probably chose to deliver it as fast as possible to help its case.