Thanks Umberto, there was a custom config for the 777-200LR and resetting it fixed the problem. The gsx.cfg file dated from 2014 and I must have made it when I first got GSX. Normally I use jetways with the 777 so I hadn't noticed that there was a problem till now.
I noticed there were other customisations in that folder which I certainly didn't make (FSL A320 and A319 for example). Are they added by the GSX updater, and if so if you reset them are they deleted?
Kevin