I think the difference is that, very generally speaking, while Beta NFs evaluate people's intentions, Gamma NTs evaluate people's intelligence. For example:
Beta NF: "You claim to be altruistically helping that person, but surely you must realize (using Ni) that this action will benefit you, so therefore you must be lying and acting selfishly."
Gamma NT: "You claim to be intelligent, working on that project, but any intelligent person like me would realize (using Ni) that your actions will have negative practical consequences, therefore you must be stupid and incompetant."
Of course there would be some overlap between the two, but I think this is the general focus of the two groups.