I know the ramp staff have to work hard, often in very unpleasant conditions, for a wage many would not consider worth getting out of bed for.
That said, the GMB should, perhaps, look back to LHR in 2004, where Swissport closed down their operation overnight, when they could not get an agreement with the unions (OK - I know there were other factors as well, and I have not got time to go into detail now, but that is the short version).
The ramp staff turned up for the early shift, only to find their passes had been cancelled, and they were out of a job. The handling contracts were shared around the remaning handlers. I`m not saying Swissport STN are in the same position, but the GMB need to be sure their members will still have a job at the finish of this dispute.