Changing "Latching Normally Closed" switch behaviour to "Momentary Normally Open"
I think the word latching is a bit misleaing in your case, because what apparently you are trying to achieve is to add some delay when the gate is open. If that is the case, this can be done by using a RC timer as follows (This circuit uses FETs but could also be implemented with BJTs).

How it works:

The gate supply voltage (V3) emulates the behaviour of the gate. The sensor and the MYSW emulates your sensor, which in this case is depedent on the gate bus (V_gate&lt;10mV -> gate and sensor are closed , V_gate>=10mV -> gate and sensor are open).

Whenever the gate is closed, the RC timer is quickly charged via M1 and D1. Once the gate is open, the timer begins to discharge at a rate given by approx. R2 ~ C1 until the FET M2 is turned off and the M3 is turned on. The diode D1 prevents the RC timer from discharging through M1. 

I have also include two outputs (NO and NC) and the waveforms can be seen right below it.