If the deck has a moulded non-slip surface you can try a GRP-safe paint remover (leave on for a few days and then pressure wash off) but I know from experience it can be a long and laborious process as deck paint is formulated to be tough resistant stuff.
I did most of my coach roof this winter with a tungstan-carbide scraper. Hard work but safe for the gel-coat.
The trick is that once you have an edge, you can work at it 5-10mm at a time even though the scraper is a lot wider. Trying to cut through a thick layer as wide as the scraper is much slower.
Sanding is fine but would have taken longer and risk going through the gel-coat. Also the paint was layed in panels so I'd have damaged the gel-coat at the edges even if I hadn't gone through. Strippers just result in a gooey mess even if they are gel-coat safe.