Only if you are using unwaxed (sanding) gelcoat. Unwaxed gelcoat stays tacky on the surface because air stops polyester from curing, so you add surfacing wax to the final coat, or spray PVA over it, to seal out air and cure it hard for sanding. Gelcoat sold with wax already contains it and cures tack-free on its own.

Waxed gelcoat has an additive that rises to the surface as it cures, sealing out air so the coat hardens tack-free and is ready to sand. Unwaxed (sanding) gelcoat has no additive, so it stays tacky until you add surfacing wax to the final coat or cover it with PVA. Use unwaxed when you plan to lay more coats, and use waxed for the last coat.

A tacky surface almost always means air-inhibited cure, not too little catalyst. Unwaxed gelcoat will not harden on top until you add surfacing wax to the final coat or seal it with PVA. Other causes are too little MEKP, temperatures below about 60°F, or high humidity. Confirm your catalyst ratio, warm the shop, and wax or PVA the last coat.