What is the Gain setting, and how does adjusting it impact tracking?

Increasing the Gain setting amplifies the brightness of the entire image and it can be used to improve the tracking at very long distances. Specifically, changing the Gain setting applies a multiplier to the brightness of every pixel in the image. 

Comparing Medium and High Gain setting from a Flex 13, the brightness of every pixel is doubled.

Increasing the Gain will result in bright regions becoming brighter by greater amounts than in dark regions. Hence, it naturally increases the image contrast (until brightness reaches the maximum value of 255), which can be useful in increasing the tracking range.

A high Gain setting should only be used to increase camera-to-marker range when modifying other camera settings (exposure setting, illumination, threshold, and f-number) are insufficient to optimize the camera image. The noise in the signal is also amplified when increasing the Gain level. Furthermore, once the brightness of background regions is amplified above the threshold level, higher Gain levels will begin to introduce noise into the 2D camera image, which can destabilize the 3D reconstruction data.