A SCS is designed to hold and secure an area of space usually a system or sector...it can either be the first ship in a battle that is to hold down the oposing force till the rest of a fleet arives or is the ship that is left behind to keep an area secure or under control (sorta like a garrison)
A GSC is just that...mainly designed as a mission specific hull to survey anomolies, systems, or planets after the inital explorer-class starships or probes first locate them...but a GSC can also be the first contact ship of its specified race...
they are mainly scienceships