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The G-force measured along the car's forward-backward axis. Positive longitudinal G indicates acceleration; negative values indicate braking. This channel shows how effectively you are using the car's braking and acceleration capabilities.
Segers defines it precisely in Analysis Techniques for Racecar Data Acquisition: "Longitudinal g-force is the acceleration logged along an axis parallel to the car's centerline (i.e., perpendicular to the lateral g-force). It is basically the acceleration created by the engine's power or the deceleration due to application of the brakes. A positive value is used for acceleration. For deceleration, the sign for the longitudinal g-force is negative." He explains reading the trace: "Maximum braking effort is displayed as the minimum value of the downward dips as the car decelerates." Under braking, a typical HPDE car on street tires generates 0.8-1.2G; under acceleration, 0.3-0.6G depending on power-to-weight ratio.