some of those GC45 models had an engine light on them and it would flash codes but if you do not have that particular system on that truck then you will need the impco software to hook up to the truck and see what codes its giving.

but it sounds to me like the throttle 'pedal position sensor' is out of range causing the code. you'll need to replace that but to set it correctly you will still need the impco program. You can try your luck doing it old school way but all i can say is good luck with that ;o)