I think at the end it comes down to what's in GSM's conditions of carriage, which if the flight was booked on the web the booker would have seen - and presumably accepted. If the conditions say "no self-provided food may be consumed" there's not much the pax can do about it. If the conditions don't say that and this e-mail instruction was sent after the booking was made, I don't see how GSM can retrospectively apply such a condition - its not enshrined in aviation law that pax can't consume their own food and such a change would effectively mean GSM were trying to vary the contract between passenger and airline. Of course GSM could always try the "you are required to comply with all instructions from air crew" line as a get out, but as eating food is hardly threatening to the AC or other pax I don't think that'd get very far if the pax ignorred that instruction.
Andy