When added to gel-coat,

• Micro-balloons will make the cured resin lighter (less dense).
• Milled glass will make the cured resin stronger.
• Fumed silica will make the uncured resin thicker so it can be spread on.
• Styrene will make the uncured resin thinner and easier to spray.
• Talc will make the cured resin easier to sand, but makes it softer too, so it's often painted over.

I hope that helps,
Eric