The gate turn off thyristor is behaves somewhat differently to a standard thyristor which can only be turned on and cannot be turned off via the gate. The gate turn off thyristor, GTO can be turned-on by a gate signal, and it can also be turned-off by a gate signal of negative polarity.
The device turn on is accomplished by a "positive current" pulse between the gate and cathode terminals. As the gate-cathode behaves like PN junction, there is a relatively small voltage between the terminals.
The turn on phenomenon in GTO is however, not as reliable as that of a standard thyristor and small positive gate current must be maintained even after turn on to improve reliability.