poor service

I have a opel opc which has a full service history i take my car to my local thorp delta to be serviced.my gearbox bearings in my box have packed up due to my gearbox oil not being changed i reported it to gm and they just said i must go to thorp.thorp inform me its not in gm checklist to replace gearbox oil.i think it is real poor service from gm and thorp that i take my car to be serviced and maintained by them and now my car has been standing for a month the bills to fix the gearbox are piling up and this all could of been avoided if it was actually maintained by the people that i pay to do it.i find gm service really poor and dnt think ill be using them again.