A gyrotron is typically challenging to simulate in 3D, as they often are electrically large. High-performance computing using one or multiple graphics processing units (GPU) can accelerate simulations significantly and make the 3D Particle in Cell (PIC) simulation of a gyrotron a viable proposition. A brief introduction will be given of a gyrotron working at the fundamental frequency, but the focus of the eSeminar will be a simulation of a gyrotron operating at 2nd harmonic frequency. The B-field can be reduced by a factor of 2, but this operation is sensitive in both the physical domain and in simulation. This eSeminar shows aspects to keep in mind when setting up the simulation and explains the differences between simulation and measurements.