There are no GMC Maintenance (Shop) Manuals at either of the links above. If I am incorrect, please post a link to a specific manual.
As Grigg posted, the "reproduction" GMC manuals are often multi-year, and the 1954/55 (Z-series) GMC wiring might be a little different, due to different dash. However, the type of gauges did not change, just the style and location of gauges. The GMC and Chevrolet Truck gauges were not at all similar in 1954/1955st trucks.
There is only a 55-59 GMC small truck Maintenance Manual at Keith Hardy's OLD ONLINE CHEVY MANUALS (no separate GMC wiring diagrams).
http://chevy.oldcarmanualproject.com/gmc/5559mm/index.html
