Regional Economic Index Posts Monthly Gain: An index designed to gauge regional economic activity six to nine months ahead posted its second consecutive monthly gain in September, according to California State University, Long Beach. The university's index of regional leading economic indicators rose to 95.8 in September from 95.7 in August. Four of six measures of economic activity--building permits, stock prices, help-wanted ads in The Times, and the U.S. index of economic indicators--rose. The university said three consecutive monthly gains in the index have traditionally augured expansion in the regional economy. The index is benchmarked at 100 in January, 1988.