I would suggest not using a chisel, it will gouge the gelcoat, get one of those plastic ice-scrapers with a solid edge. Unfortunately there is no short-cut to this type of miserable task, you just have to git-r-dun!!! You can try soaking a small area with acetone, let it sit for a while and then try removing the non-slip rubber. Once the majority of the rubber has been removed you can hit it with the goo-b-gone.