Here are the steps to set IDAC gain:
1) In PSoC Creator, navigate to Tools -> Options ->Design Entry-> Component Catalog and enable Param Edit Values
2)Then right click on the Advanced Configuration Tab of CapSense component and click on Show Expression View.
This will open a window. Scroll down to CSD settings and set appropriate gain in Csd0IdacGainV2.
DISCLAIMER: Do not use the expression view for other purposes/change any other parameters, as it may affect the CapSense performance.