Via <setdcgasdefinitionsdata/> the breathing gas definitions are transferred to the dive computer.
(see also <setdcdata>)
<divecomputercontrol>
    <setdcdata>
        <setdcgasdefinitionsdata>
            <!-- send all defined breathing gases to the dive computer -->
            <setdcallgasdefinitions/>
        </setdcgasdefinitionsdata>
    </setdcdata>
</divecomputercontrol>
                
<divecomputercontrol>
    <setdcdata>
        <setdcgasdefinitionsdata>
        <!-- send only selected breathing gases to the dive computer -->
        <setdcgasdata ref="pure_oxygen"/>
        <setdcgasdata ref="air"/>
        <setdcgasdata ref="my_trimix"/>
        </setdcgasdefinitionsdata>
    </setdcdata>
</divecomputercontrol>