As stiff brush may leave "slight" scratches (hard to see on white), but not likely.
I wouldn't worry about any plastic type brush like a scrub brush, but the best bet is just test an inconspicuous area with whatever device or solution you're using if you have any doubt.
Gel coat is tough stuff.
Also an orange based cleaner/degreaser may work well and simple green sounds like something good to try.
If it's just crusted on organics, time for the cleaner to penetrate/soak in and soften will help too.
A great, but somewhat expensive, spray and wash away cleaner is cycle wash, for dirt bikes. That stuff is pretty awesome. It penetrates baked on dirt/scum and is also a mild de-greaser.