They have gaped upon me - Changing the form from the singular to the plural, and including “all” his pretended friends. Such a change in the number is not uncommon. His mind seems to have passed from the particular instance which he was contemplating, to “all” his friends, and he suddenly felt that “all” had treated him alike. The meaning is, that, like wild beasts, they open their mouth to devour me.
They have gathered themselves together - They have entered into a conspiracy, and have “agreed” to oppose me. They are united in this thing, and all feel and act alike.