  We present a numerical implementation of the guiding center approximation to
describe the relativistic motion of charged test particles in the PLUTO code
for astrophysical plasma dynamics. The guiding center approximation (GCA)
removes the time step constraint due to particle gyration around magnetic field
lines by following the particle center of motion rather than its full
trajectory. The gyration can be detached from the guiding center motion if
electromagnetic fields vary sufficiently slow compared to the particle gyration
radius and period. Our implementation employs a variable step-size linear
multistep method, more efficient when compared to traditional one-step Runge
Kutta schemes. A number of numerical benchmarks is presented in order to assess
the validity of our implementation.
