  At least a fraction of Gravitational Wave (GW) progenitors are expected to
emit an electromagnetic (EM) signal in the form of a short gamma-ray burst
(sGRB). Discovering such a transient EM counterpart is challenging because the
\LIGO/\VIRGO localization region is much larger (several hundreds of square
degrees) than the field of view of X-ray, optical and radio telescopes. The
Fermi Large Area Telescope (LAT) has a wide field of view ($\sim 2.4$ sr), and
detects $\sim 2-3$ sGRBs per year above 100 MeV. It can detect them not only
during the short prompt phase, but also during their long-lasting high-energy
afterglow phase. If other wide-field high-energy instruments such as Fermi-GBM,
Swift-BAT or INTEGRAL-ISGRI cannot detect or localize with enough precision an
EM counterpart during the prompt phase, the LAT can potentially pinpoint it
with $\lesssim 10$ arcmin accuracy during the afterglow phase. This routinely
happens with gamma-ray bursts. Moreover, the LAT will cover the entire
localization region within hours of any triggers during normal operations,
allowing the $\gamma$-ray flux of any EM counterpart to be measured or
constrained. We illustrate two new ad hoc methods to search for EM counterparts
with the LAT and their application to the GW candidate LVT151012.
