  The X-ray plateau emission observed in many Long gamma-ray bursts (LGRBs) has
been usually interpreted as the spin-down luminosity of a rapidly spinning,
highly magnetized neutron star (millisecond magnetar). If this is true, then
the magnetar may emit extended gravitational wave (GW) emission associated with
the X-ray plateau due to non-axisymmetric deformation or various stellar
oscillations. The advanced LIGO and Virgo detectors have searched for
long-duration GW transients for several years, no evidence of GWs from any
magnetar has been found until now. In this work, we attempt to search for
signature of GW radiation in the electromagnetic observation of 30 LGRBs under
the assumption of the magnetar model.
  We utilize the observations of the LGRB plateau to constrain the properties
of the new-born magnetar, including the initial spin period $P_0$, diploe
magnetic field strength $B_p$ and the ellipticity $\epsilon$. We find that
there are some tight relations between magnetar parameters, e.g., $\epsilon
\propto B_p^{1.29}$ and $B_p\propto P_0^{1.14}$.
  In addition, we derive the GW strain for magnetar sample via their spin-down
processes, and find that the GWs from these objects may not be detectable by
the aLIGO and ET detectors. For a rapidly spinning magnetar ($P\sim 1\mbox{
ms}$, $ B \sim10^{15}\mbox{ G}$), the detection horizon for advanced LIGO O5
detector is $\sim 180\mbox{ Mpc}$. The detection of such GW signal associated
with the X-ray plateau would be a smoking gun that the central engine of GRB is
a magnetar.
