I also failed to find the gaskets for in-line connections of gutters, so I
used Henry's Roofing Patch. Have to renew it every year or two, but it
works. Just clean and dry joint thoroughly, then use a putty knife to press
the compound into the gap, from the inside of the gutter. Unfortunately the
gaskets for plastic gutters are crappy quality, and weather, then stiffen
and shrink/dislodge with age, and subsequently leak.