  Much excitement has been generated recently by the experimental observation
of the chiral anomaly in condensed matter physics. This manifests as strong
negative longitudinal magnetoresistance and has so far been clearly observed in
Na$_3$Bi, ZrTe$_5$, and GdPtBi. In this work we point out that the chiral
anomaly must lead to another effect in topological metals, that has been
overlooked so far: Giant Planar Hall Effect (GPHE), which is the appearance of
a large transverse voltage when the in plane magnetic field is not aligned with
the current. Moreover, we demonstrate that the GPHE is closely related to the
angular narrowing of the negative longitudinal magnetoresistance signal,
observed experimentally.
