Depending on your source, the neighborhood of Belvidere is either an area north of SW Manning St, south and west of the Duwamish Head greenbelt, and east of Fairmount Gulch, centered around the eponymous avenue; or it's a made-up name that real estate agents use to list houses in the eastern portion of the North Admiral neighborhood. The official City of Seattle neighborhoods map considers it the latter, while people who live in the neighborhood seem to prefer to retain a separate identity based around the quieter tone of the neighborhood, the beautiful views of downtown Seattle to the north, and the physical separation from the adjoining neighborhoods to the north, east and west due to steep ravines creating a natural demarcation.
You're looking for a magnetic keyholder at a very popular viewpoint, in a spot that might be very familiar to long-time cachers. The totem pole at this viewpoint, installed in 1966, has a much less sordid history than the one at Rotary Viewpoint a little over a mile south. Depending on the time of year the foliage might block the best views from GZ, but that could be a benefit as the muggles will congregate elsewhere.