Sanded off all the old graphics & can now really see difference between the gel coat that was under the paint, & the majority that is now sun faded. What is the best way to bring the white back out? I really don't want to have to end up painting the whole boat.

Sand lighter! Water sand with lets say... 500 then 600 then polish, it should come back like new! Of course i dont see how it looks now but you get the idea! Rougher towards softer. Should be good!

You'll never get the exposed white gel to be as white as what was under the paint. You can try wet sanding and it will probably help some.
I'd say maybe go light with 600 and then 1000 and then buff with a heavy cut 3M compound.

I still don't think it will get white enough to match what was under the painted parts.