Tree sap is hardened non-polar resin most like turpentine. It will dissolve in a similar substance- like turpentine- that will not hurt polyester resin/gel coat. But turpentine (distilled from pine tree stumps) is now very scarce except in an art supply store. Use mineral spirits (or charcoal fire starter) to soften and dissolve it the sap. This takes a while for hard sap, even using a pad full of solvent sitting on it. Do NOT use lacquer thinner, acetone or MEK.

SIgned, Ph.D. Organic Chemist