PFUG means pre flight upgrade ie they had already decided to upgrade you before even got to the airport (at Heathrow at least this is normally accompanied by someone from Special Services coming down to say 'hello')

My understanding is that for the past couple of months they have been upgrading selected gold card holders.

Very occasionally you will also see IVUG which I think means involuntary upgrade - but I am not really sure what the difference between the two is.