GSS only ever existed to operate BAWC services and was only formed as a UK subsidiary of Atlas when it was pointed out by the CAA that it was illegal for an N-reg company to carry out UK based work. I think BALPA were quite concerned about US pilots helping themselves to other people's commands.
BALPA had nothing to do with it. It was the IPA who pressed the case. BAWC had been stalling the issue - apparently on the back of BALPA's indifference. But the British Airways Line Pilot's Association got their own back with the secondee commands.