Looks like an 86 serial number but made overseas and not in the US which is where all the complete bikes such as the World Tour etc came from. They normally have another letter after the '6' though.
I have had a few and seen a few from 85 with the following serial numbers: G5C****, G5D****, G5F****, G5J****
I think this typically translates to:
G - Overseas manufacturing code or Frame ref? (maybe for GT)
5 - Year 85
C - Month (March)
**** - Serial #