  Recent work has shown that modified gravitational wave (GW) propagation can
be a powerful probe of dark energy and modified gravity, specific to GW
observations. We use the technique of Gaussian processes, that allows the
reconstruction of a function from the data without assuming any
parametrization, to measurements of the GW luminosity distance from simulated
joint GW-GRB detections, combined with measurements of the electromagnetic
luminosity distance by simulated DES data. For the GW events we consider both a
second-generation LIGO/Virgo/Kagra (HVLKI) network, and a third-generation
detector such as the Einstein Telescope. We find that the HVLKI network at
target sensitivity, with $O(15)$ neutron star binaries with electromagnetic
counterpart, could already detect deviations from GR at a level predicted by
some modified gravity models, and a third-generation detector such as ET would
have a remarkable discovery potential. We discuss the complementarity of the
Gaussian processes technique to the $(\Xi_0,n)$ parametrization of modified GW
propagation.
