You can use gb << Show Tree Structure() to give idea of the elements and with that you can usually get idea how to access specific element
In this case you can access LegendBox this way:
gb_report = gb << Report;
legend = gb_report[legendbox(1)];
and set property most likely with legend << Enabled(0)
Full example:
Names Default To Here(1);
dt = Open("$SAMPLE_DATA/Big Class.jmp");
gb = dt << Graph Builder(
Show Control Panel(0),
Variables(X(:sex), Y(:height), Group X(:age)),
Elements(Box Plot(X, Y, Legend(1)))
);
gb_report = gb << Report;
legend = gb_report[legendbox(1)];
legend << Enabled(0);
-Jarmo