There is no doubt that some twit a**e licking CSD's did chuck out bad GPM forms in the past, frightened and intimidated that if their personal scores dropped because the IFE packed up or the aircraft was delayed, that somehow they were responsible for it, and would get 'managed' by their CCM over it. A couple of CSD's were sacked over 'GPM fraud.'
Most CSD's put them all in as they wanted the bad comments to reinforce the problems they encounter on every flight. Of course in spite of the constant evidence from passengers, BA never corrected what passengers most complained about.
Strangely BA put huge stock in the GPM's following trends when in fact the results were not always accurate, as the passengers who really should complete them would often refuse. For a long time now the forms can be sealed by the passenger.