You will probably remove more weight in preparing the existing gelcoat for paint than two pack polyurethane will add. The two pack covers incredibly well and provides a surface that is much harder than the gelcoat it covers. Even when rollered and tipped off the paint coat is unbelievably thin (you lose solvent and hence weight during curing). Prepare well and do the job properly first time, DON'T SKIMP. Sanding off a coat of two pack is several times more difficult/time consuming than sanding gelcoat.

Cheshirecatman