This is my first time working with gell but I will offer my two cents.
First,gellcoat is not something that should be attempted unless you have bodyshop experience and or a serious drive to get the job done.As an example I have over a hundred hours in wetsanding.You will also need a handfull of expensive tools.
Second,its not very healthy and care should be taken not to poison yourself as best you can.
I found that a standard sanding block doesn't work well on the straight sides of a boat and remebered a old trick from my bodyshop days.This will also work if your sanding you existing gellcoat.
Go to your local parts store and buy a roll of campertop foam seal then cut a wood block the length of sanding paper\width of seal.apply a piece of foam to one side and you have a rigid sanding block thats easy to hold onto without sanding your fingers off.Split a piece of paper in half then fold it and your ready to go.
I also use a rubber sqweegee to clear water away and inspect.Dish soap in the sanding water also helps a bit.
When sanding place the block at a slight angle and make long clean strokes making sure to stay off the edges.
When you are satisfied with a area use the sqweegee to mark your stopping point.
I started with 400 grit and then on to 1000 but everyone has a different system.Do what works best for you just remember whatever you start with you have to sand out later.Keep in mind too the longer you let it dry the harder the gell will get so start sanding as soon as possible.
I used about thirty sheets of 400 and twenty on 1000.