VO2223A datasheet query -- pin 5
Often, gate connections on photootriacs (and phototransistors) are used to:


Turn on the device separately from the LED (a remote trigger function)
Hold the device on once the stimulus goes away (a latch function)


I would leave the gate floating, with an option for a tie-down resistor in your layout. Most of the time, you need to supply current to the gate in order to activate the device - tying the gate high will most likely turn on the triac all the time, which is something you don't want.