The G & I-Click devices share the same control box and outputs, however, their sensors differ and the principal difference is that the G-Click gyroscope sensor works best at detecting articulated movement from joints or a tilt from any part of the body.   While the I-Click has an Infrared sensor that is designed to detect very slight changes in the distance from the sensor suitable for detecting twitches from muscles, typically on the face but it could be any muscle.