Analog Input >> General Properties >> Advanced >> Gain >> Gain Value

NI-DAQmx C Properties

Analog Input >> General Properties >> Advanced >> Gain >> Gain Value

Data Type: float64
Description: Specifies a gain factor to apply to the channel.

You can get/set/reset this property using:
  DAQmxGetAIGain
  DAQmxSetAIGain
  DAQmxResetAIGain