The GAIN setting allows for fine-tuning the dive computer's data reception frequency from the transmitter.
The principle is the same as the fine-tuning used in FM communication radios.

In the first gen of the iX3M the gain has to be set manually (also during the dive) following this tutorial:
https://www.ratio-computers.com/support/support4/how_to/video_ix3m_set_gain_underwater.htm

In the second gen of iX3M2, the dive computer automatically sets the Gain.