THE GATES by Chuck Wachtel. (Viking: $23.95; 402 pp.) For the first 120 pages of "The Gates," protagonist Primo Thomas is bored and alienated. So is the reader. This, presumably, is by design. Wachtel's villain--a popular one in a progressively cynical world--is indifference, anomie, a vague but persistent malaise. These are difficult topics to deal with, to read about. In Primo Thomas' case, the epiphany is worth the wait. Primo's father was a black doctor, his mother a white Italian whose family has cut her off. Primo himself, 37, is divorced. After two years as a instructor in Boston, he returns to his native New York, where he teaches English to adult immigrants. Nothing bothers him; everything bothers him. With a dozen other teachers, he goes to Nicaragua to tour educational facilities. The Sandinistas are trying to rebuild their country, post-Somoza (it's 1988). The Contras, with the help of the U.S. military/industrial complex--are sabotaging their efforts. ("The Gates" is nothing if not PC.) In Managua, Primo meets Angelita, embodiment of individual responsibility, sacrifice, one-person-can-make-a-difference. Primo not only listens, he hears. He is transformed. Back in New York, he determines to make his difference. If this sounds sappy, simplistically moral, it's not. Wachtel, while sometimes stiflingly introspective, is a fluid modern Aesop. There is enough light banter, moments of sheer wackiness, to effectively relieve the gloom. Yes, there is a moral, but one well worth investigating.