What kind of GPS do private pilots use when flying a cessna 152 or 172?

An automotive GPS is almost useless for flight. Seeing turn-by-turn street directions is not going to be useful to you while flying, and the GPS will be constantly rerouting because it sees that you're not following the directions it's giving you. It also won't have any of the map data necessary for flight, such as VOR locations (yes you still use VOR locations when flying by GPS), airspace boundaries, instrument approaches, or other helpful information like radio frequencies and runway details. Modern Cessna 172 aircraft frequently include the Garmin G1000, which combines GPS and provides a glass cockpit for display of most major gauges. The recent Cessna 162 includes a less expensive Garmin 300, but still has similar general capabilities. There are also portable Aviation GPS units that can be plugged into a 12V outlet and stuck on top of the "dashboard", such as the Garmin GPSMAP 196. There are other aviation GPS vendors... Another common general aviation vendor is Bendix-King. Of course you also need to purchase map updates for your GPS, which are far more important for aviation than for automotive use (Sectionals maps typically expire every 6 months, and the GPS data updates come out in coordination with them). If you have an iPad 3G, then consider getting the "ForeFlight" app which has full GPS and sectional support. It's actually used by many general aviation professional pilots for navigation purposes.

They use a GPS made for aircraft use, not one made for automobile use. The maps and databases aren't the same. Here is a popular one: http://www.aircraftspruce.com/menus/av/gps_handheld.html