  The Fermi Gamma-ray Burst Monitor reported the possible detection of the
gamma-ray counterpart of a binary black hole merger event, GW150914. We show
that the gamma-ray emission is caused by a relativistic outflow with Lorentz
factor larger than 10. Subsequently, debris outflow pushes the ambient gas to
form a shock, which is responsible for the afterglow synchrotron emission. We
find that the 1.4 GHz radio flux peaks at $\sim10^5$ sec after the burst
trigger. If the ambient matter is dense enough with density larger than
$\sim10^{-2}$ cm$^{-3}$, then the peak radio flux is $\sim0.1$ mJy, which is
detectable with radio telescopes such as the Very Large Array. The optical
afterglow peaks earlier than the radio, and if the ambient matter density is
larger than $\sim0.1$ cm$^{-3}$, the optical flux is detectable with large
telescopes such as the Subaru Hyper Suprime-Cam. To reveal the currently
unknown mechanisms of the outflow and its gamma-ray emission associated with
the binary black hole merger event, follow-up electromagnetic observations of
afterglows are important. Detection of the afterglow will localize the sky
position of the gravitational wave and the gamma-ray emissions, and it will
support the physical association between them.
