Example of Setting Auto Gain

Hello, I am not sure how to set auto gain on a RTL dongle. I ran across this which I think should be what I need: http://cgit.osmocom.org/gr-osmosdr/tree/grc/gen_osmosdr_blocks.py From that page: |<param> <name>Ch$(n): Gain Mode</name> <key>gain_mode$(n)</key> <value>False</value> <type>bool</type> <hide>\#if \$nchan() > $n then 'none' else 'all'#</hide> <option> <name>Manual</name> <key>False</key> </option> <option> <name>Automatic</name> <key>True</key> </option> </param> | However it is not clear what I need to pass to make auto gain happen. Thanks, John