Nikkei on track for biggest daily gain since June 2013 on upbeat U.S. data, GPIF

* News GPIF's raising Japan stock allocation to 25 pct underpins sentiment * Investors scoop up battered exporters as yen weakens * Gains may be short-lived as market still spooked-analyst By Ayai Tomisawa TOKYO, Oct 20 (Reuters) - Japan's Nikkei share average surged 3.3 percent on Monday and was on track to post its biggest daily rise in more than a year as investors took heart from upbeat U.S. data and as the weaker yen lifted exporters such as Toyota Motor Corp and Honda Moto