Small Gain in Ad Spending: Spending for newspaper advertising rose 6.1% to an estimated $31.2 billion in 1988, the smallest percentage gain in 13 years, the Newspaper Advertising Bureau Inc. said. The increase was less than a 9% rise in 1987 and was the smallest advance since a 5% gain in the recession year of 1975. Preliminary estimates indicate that spending for retail advertising, the largest category for newspapers, rose 4% to $15.84 billion last year. Classified ad spending rose 10.1% to $11.77 billion, and national ad spending was up 2.6% to $3.59 billion. Media analysts have blamed the sluggish growth in retail ad spending partly on recent consolidation in the retailing business.