I would say gel-coats tend to be fairly hard, especially modern gel-coats as technology is always changing. I've worked on some pretty old gel-coat hulls and they always seem to cut a littler easier than anything made in the last 20 years or so...
If you look at Marine compounds, they also tend to be a lot more aggressive than automotive products and there's a number of reasons for this...
If you're just trying to remove light oxidation you might be able to get away with a DA Polisher with a foam polishing or cutting pad and strong cleaner/wax.
If you're trying to chop off heavy oxidation and staining then the fastest way to do it is with a wool pad on a rotary buffer with an aggressive cutting compound.
Sometimes wetsanding is the best option, I sanded down the sides of my hull to remove years of oxidation and faded gel-coat, came out pretty good.