  Observations of extended gamma-ray emission around Galactic cosmic-ray (CR)
sources can be used as novel probes of interstellar magnetic fields. Using
very-high-energy gamma-ray data from the HAWC Observatory, we place constraints
on the properties of the magnetic turbulence within $\approx 25$ pc from
Geminga. We inject and propagate individual CR electrons in 3D realizations of
turbulent magnetic fields, calculate the resulting gamma-ray emission, and
compare with HAWC measurements of this region. We find that HAWC data is
compatible with expectations for Kolmogorov or Kraichnan turbulence, and can be
well fitted for reasonable coherence lengths and strengths of the turbulence,
despite implying a CR diffusion coefficient significantly smaller than those
suggested by Galactic CR propagation codes. The best fit is found for a
coherence length $L_{\rm c} \approx 1$ pc and a magnetic field strength $B_{\rm
rms} \approx 3 \mu$G, and the preferred value for $L_{\rm c}$ increases with
$B_{\rm rms}$. Moreover, the apparent lack of strong asymmetries in the
observed emission allows us to constrain the coherence length to $L_{\rm c}
\lesssim 5$ pc in this region.
