When placing a sensor near the gates, the electron will not behave like a wave anymore upon entering. It will behave like a particle and we will, instead of an interference pattern, just have two lines denoting which gate the particle went through.
Unlike with electrons behaving like waves where the squared absolute value encompasses the sum of the amplitudes, for electrons behaving like matter the squared absolute value acts on both amplitudes seperately. As such the intenity pattern can be found by calculating the sum of the squared absolute value of the amplitudes.