There are several different additives available for gel coats. They each have a unique purpose. MEKP is the only one that is required. It is the catalyst for polyester resin, vinyl ester resin and gel coats.
Also available are additives that thin the gel coat for spraying, create a tack free surface or make different visual effects. You will want to make sure you read the instructions (technical data sheet) to know how much to add.