I'm puzzled by this line of reasoning. Distrust veganism by all means, but not because of the poor choices, priorities, or sad family life of someone who promotes it.
Have a read of The China Study, and pick it apart. Eat a whole food, plant based diet yourself for 3 months and tell us how you feel.
Those are much better ways of testing this way of eating than the life story and supposed financial affairs of Dr Gregor.