In his paper Gatheral presents the following parametrization of the implied total variance $w(k,T) = \sigma_{BS}(k,T)^2T$
$$ w(k) = a + b\{\rho (k-m) + \sqrt{(k-m)^2 + \sigma^2} \}.$$
Assuming that we only have a few market prices e.g. 6 or 7 which are close to at-the-money. I wanted to know if there are any common techniques to extrapolate the implied volatility for Strikes that are far out-of-the-money.