Based on finite-difference time-domain simulation of the propagation characteristics of surface plasmon polaritons (SPPs) in optical circuits made from metal gap waveguides (MGWs) with nanometric gap widths, we theoretically demonstrate that two structures that consist of splitting and recombining MGWs and of coupling MGWs can be used as nanoscale Mach-Zehnder interferometers. MGW arrays show capabilities for array imaging and for controlling the flow of SPPs. Other potential applications of coupling MGWs, as SPP switches, directional couplers, and even as a nanoscale counterpart for observing linear and nonlinear dynamic behavior of electromagnetic fields, are also predicted and discussed. Our results point to an interesting way to manipulate optical signals and provide efficient sensing in nanophotonic architectures.