Stock prices eked out a small gain today as the market absorbed the news of increases in the prime lending rate. The Dow Jones average of 30 industrials rose 4.98 to 2,109.35 by 3 p.m. on Wall Street. Gainers slightly outnumbered losers in the overall count of New York Stock Exchange-listed issues, with 708 up, 663 down and 545 unchanged. Many large banks across the country raised their prime lending rates today from 9 to 9.5 percent. Analysts said, however, that those moves were seen largely as a response to recent increases in open-market money rates and did not necessarily have much predictive value for the future course of rates in general. Alan Greenspan, chairman of the Federal Reserve Board, said Wednesday that further increases in rates were not inevitable. In any case, brokers said traders were reluctant to commit themselves extensively either way in advance of Friday's report from the Commerce Department on the nation's trade balance for May. Millipore climbed 1| to 38~ and Worthington Industries, traded in the over-the-counter market, gained 1[ to 24]. The two stocks were added to Standard & Poor's 500-stock composite index, prompting buying by index funds set up to duplicate the performance of the 500. Motorola fell 1} to 50{. The company reported second-quarter earnings of 93 cents a share, up from 62 cents in the comparable period last year but below what some Wall Streeters had been looking for. The NYSE's composite index of all its listed common stocks rose .21 to 152.48. At the American Stock Exchange, the market value index was up .19 at 307.85. Volume on the Big Board came to 147.41 million shares with an hour to go.