to get the goods

no, the cameramen are going to secretly film or videotape somebody doing something illegal or unethical. the film will then be used as evidence to accuse the person caught on film or videotape of wrongdoing.

"Proving" lawbreaking or marital infidelity needs evidence, hard evidence.

The cameraman you mention is going to record proof, "the goods" of illegal action.

hence the goods equals the evidence, right?

Only in that particular case. In the expression "get the goods," goods means evidence. Otherwise, it just means merchandise.