I'm interested to hear what others say, but right on the box there is a disclaimer:
"Disclaimer: Bunk Wrap is intended for Aluminum boats. If hauling a boar with a gel coat hull, certain conditions may scuff your gel coat. If you are concerned about scuffing your gel coat hull use Caliber Slydz-On (Part#23200) and carpet to achieve reduced friction."
But also on their website under the faq's is this:
" Q: Will Marine Slides scratch my hull?
A. After extensive testing of all brands of marine slides we have found that all slides will, under some conditions, scratch your Boat/Watercraft hull. This usually occurs when the slides or hull is dirty or sandy.
It is a common misconception that carpet (used on most trailer bunks) will not scratch your boat. Carpet in-fact will scratch your boat with fine sand particles that are embedded in the carpet. Carpet is however good at dispersing gel-coated material (widely used to manufacture boat hulls) so the owner does not see that his carpet is in fact harming his/her hull. Conversely, Marine Slides will show signs of material being deposited on them from your hull."
so... pick your poison?