Technically yes, the GAIN does lower and raise the boost. But the main setting for increasing boost is 'SET'
This is what the manual says...
Adjusts boost consistency, if you have boost spikes lower the GAIN, if the boost falls off at high rpm or the boost gradually increases as rpm increases, raise the GAIN.
Find the full manual in PDF here
http://www.greddy.com/tech/PROFEC_B_SPEC2.pdf