What would Gandhi have thought? The Oscar Award-winning movie commemorating the career of Mohandas K. Gandhi is to open later this month in South Africa . . . to segregated audiences? Monitor correspondent Paul Van Slambrouck reports that critics of the policy have urged the government to use the showing of ''Gandhi,'' which depicts some of the Indian lawyer's early days in South Africa, as a good occasion to abolish the practice. That is unlikely. But a government official has raised the possibility that at least the premiere of the show could be opened to a multiracial audience.