Unless you do as Nick suggests, and pay someone else, there is no easy way.
Using sandpaper is out-of-favour and often not permitted in many yards due to the likelihood of toxic dust spreading about the yard. You can tarp off your boat, but then you are putting yourself in that toxic cloud that concentrates as you work.
Wetsanding avoids the dust, but creates a runoff that must be contained and disposed of as well. In these environmentally concious times you just can't get away with much anymore.
Scrapers, dropping the product onto a ground sheet so you can collect it will do the job with a mimimum of mess and carry-over. A gel - type paint remover can be used for early layers, but caution is needed when you get close to the actual fibreglass.
Other than a full gelcoat peel, it's going to take either cash or a lot of elbow grease to get to where you want to be.
I won't say: Have fun....