Thanks David. The method you described applies to Piaware SD card image, but @send2gl has Piaware Add-on package, and in case of Add-on package, the command
piaware-config rtlsdr-gain 30
will change setting only in piaware config file, but NOT in dump1090-fa
Please see this new thread I have created few minutes ago which covers both types of installs (i.e. Piaware SD Card and Piaware Add-on)