I have an 1998 GL1500.
Reverse on this model as with all the earlier one's, from 1988, has the reverse lever to the left side of the "dummy" tank which is required to be pulled vertical from its usual horizontal position, with the gear box in neutral, then hit the starter button. The bike crawls slowly backwards until you release the starter. (or flatten the battery, I have been told it will do away 250 yards before this happens, but have never been game to try. (Why would you).
Once reverse is completed the lever is returned to the horizontal and all is ready to continue as normal.
Brett