The stock market eked out a modest gain today in light pre-Christmas trading. Wall Streeters were expecting a quiet day as many investors opted to get an early start on the long holiday weekend. Brokers also said money managers at investing institutions had pretty much finished their maneuvering for the year. However, stocks did attract a few nibbles from buyers as the market opened. There has been a lot of talk lately of accumulating stocks before New Year's to try to benefit from the so-called January effect. Academic researchers have published many studies about an evident tendency of stock prices to rise in the first month of the year. Whether that phenomenon will persist in 1989, of course, remains to be seen.