In the above circuit is we are applying high voltage in any one of the diodes or in both diodes simultaneously then we get a high out due to forward biasing of diode while we are applied low voltage in both the diode then we get low voltage so this is like OR gate.
In the above circuit which is an NPN transistor in a common emitter configuration having a phase difference of 180∘ between input voltage and output voltage, when we are applied high voltage then we get low voltage vice-versa so, the circuit is the same as NOT gate.