You cannot reason with Gerald - I worked with students on the autistic spectrum for many years and once they had made up their minds about an issue it was virtually impossible to get them to change their opinion.

You will not get Gerald to debate or discuss his views. He will, however, repeat minor variations of the same material for years on an almost daily basis.

Gerald cannot see situations from any other viewpoint than his own. This is a characteristic of his condition. He neither knows nor cares if he is annoying or upsetting other users of the group.

Posting to the group is an important part of his life. It is hard to imagine that he is in gainful employment or that he has a "normal" social life so he has become a "keyboard warrior" instead.

The solution is simple. Ignore him. Never respond to his posts, turn him into a non-person.