[roz1]

Actually, I believe that accelerometer and G-sensor refer to the same thing.

G-sensor is a marketing-type name for accelerometers. An accelerometer can measure linear acceleration such as the force of gravity pulling it down (that's how it knows your orientation). This where they get the "G" from, or similarly G forces that a fighter pilot would feel. As Miami Son mentioned above, not all accelerometers are created equal, but most new phones have pretty decent accelerometers since they are now relatively cheap.

Gyroscopes measure the rate of rotation (i.e. twisting motions).

The TP2 has an accelerometer.
The iphone has an accelerometer. If I'm not mistaken, the iphone 4 also adds a gyroscope.