Any paint stripper will handle the varnish, and won't damage the gelcoat. I'd just make very sure that it IS gelcoat before I starting slopping stripper around.
The sealant's gonna be a lttle different. I'd first try to carefully remove it with a razor blade scrapper. Depending on what type of sealant you're dealing with, you may need to resort to chemical warfare. I've had luck getting polysulfides off with acetone and PB Blaster. 3M General Purpose Adhesive Remover may also work. If it's 3M 5200, you'll probably need to resort to DeBond 2000.