What is the relation between the gain and range of a data-acquisition amplifier?
You are fixated on a completely irrelevant concept. There is no way to tell what the internal gain of the board is, and no earthly reason to care.

If your input data range is more than +/- 5 volts, select for a +/- 10 volt range. If your input range is less than +/- 50 mV, select the +/- 50 mV range. Since you have no way to tell the internal signal levels, gain is irrelevant.

Determine your input signal levels and select the appropriate range. Forget about gain.