Before revision:  Based on that, a gamma distribution with matched skewness-kurtosis ratio is proposed to approximate its distribution. Comparing with existing methods, the new method is significantly more accurate in getting the right-tail probability. The new method may facilitate the hypothesis testing in the analysis of big data , where efficient and accurate calculation of  small p-values is desired.
After revision:  Based on that, a gamma distribution with matched skewness-kurtosis ratio is proposed to match the ratio of skewness and kurtosis based on the gamma distribution. Our extensive simulations show that 1) MR is almost as accurate as the exact distribution calculation and is much more efficient; 2) comparing with existing approximation methods, MR significantly improves the accuracy of approximating far right tail probabilities. The proposed method has wide applications. For example, it is a better choice than existing methods for facilitating hypothesis testing in the analysis of big data , where efficient and accurate calculation of small p-values is desired.