i have a device simular to this called the ladder basemate stabiliser, it works on the same concept of holding the bottom two rungs
i used it on wet decking once and the ladder slipped when i was at the top, the ladder stayed upright but came about a foot away from the window sill and stayed there in mid air, i crapped myself
that device would do exactly the same, there not 100percent but i would look at having one as the ladder angle is adjustable for doing gutter clears