If I am using an inductive proximity sensors as a safety gate latch ...
... when the metal door opens the inductive proximity sensor will trigger.
I'm concerned that you may not understand the potential failure modes.
Safety integrity level (SIL) is defined as a relative level of risk-reduction provided by a safety function, or to specify a target level of risk reduction. In simple terms, SIL is a measurement of performance required for a safety instrumented function (SIF). Source: Safety integrity level.
You need to perform a risk analysis based on
- Severity of possible injury.
- Frequency of exposure.
- Likelihood of avoidance.
That will determine the SIL required.
In general, an inductive proximity switch would not be suitable. You can't predict its failure mode - open-circuit or short-circuit and therefore you can't guarantee that it will switch off the hazard when "triggered" - whatever that means.
Instead, the SIL will require that the switch output turns off when the guard is opened - either by forced disconnection of the switch contacts or by some other means. If there is a risk of permanent or "non-reversible" injury or death the system will require redundant or dual-channel safety with a safety controller that will guarantee that a fault on one channel is detected and will not reset until the fault is cleared.
Your question and comments suggest that you are going to use a prox switch to detect opening of the gate. Instead your safety must prove that the safety gate is closed before energising the circuit.