My deck has trex-style pond liner hung between each of the joists that feed into a gutter below. Surface tension causes lower flow to wrap around the edge and shoot past the gutter.
These are meant to clip on to the edge of each gutter end to help guide the water flow. There is a small hole where you can hang a printable drain guide.
There are two angles of drain guides attached.
The author marked this model as their own original creation.