If you are going to be hiking in Hong Kong, you may need hiking maps, but a Google map is not going to help you, as the trails are not going to show up on the Google maps. Or at least they will not show up in any detail that would be useful to you, as you won't be able to get close-up views of the trails; in my experience the most you would see is the broad outline of a dirt trail along a ridge line (you can see parts of the Dragon's Back trail if you Google the Shek-O area for example.)

But virtually all trails have maps at various points (esp at the trail heads) and have signage at various points along the trail as well. The trails for the most part are not complicated, and have few forks or intersections, and those that do are marked so you know which way to go to follow the trail you want. And unless you are walking on a weekday, you are bound to run into a few other hikers on most trails, so you can always ask. (On popular trails on Hong Kong Island you may run into hikers on weekday as well.) I don’t see any reason to use a Google map, esp as you don’t know the area to begin with. (Also note that on Google maps, many of the names may only be in Chinese.)

What you may find useful are the “Countryside Series” of maps for each area of Hong Kong. There is one for Hong Kong Island, one for Lantau, and three for the New Territories as they are divided up into Sai Kung, and then the East NT and West NT. These maps are usually available at Dymocks, a local chain of bookstores here, go to http://www.dymocks.com.au/contentstatic/stores/region_Hong+Kong.asp (or just dymocks.com.au) for a list of locations; you also may be able to order via the web from them and have them mailed to you. Dymocks usually has a very good selection of walking books generally. The maps are definitely available at the Government Publications Office, located in the Murray Building, Room 402, 22 Garden Road (tel 2537-1910). This is an office building right next to the bottom station for the Peak Tram. They are open 9-6 pm Mon-Fri.
The Hong Kong Tourist Board has a free guide which you can pick up at any of their offices called “Exploring Hong Kong’s Countryside: A Visitor’s Companion” which has descriptions of walks and maps; if you are stopping into one of the HKTB offices this would.

There are numerous walking/hiking trails all over Hong Kong Island proper. I can think of about half a dozen right now without even looking at my map. There are several on and around the Peak, including the Green Trail which you can take up or down from Central to the Peak. You can walk from the skyscrapers of Central to Stanley and Shek-O beaches entirely on public trails.