Guttering joiner patchs, questions????

Are you talking about right at the corner? Did they actually have a piece there? When I took my trim off, there was nothing at the corner, but my car was repainted long ago, though I don't think they bothered to remove all the trim.
Unless someone offers an alternative, I would weld something in, then use something waterproof to fill/smooth, like JB Weld, then paint.