To get a gold medal in your missions you must earn the required amount of points and still have all your cores at the end of the mission. If you find yourself short a couple points during the endgame results, remember that the amount of resources you have add to your points, so if you can build a tower or upgrade less, then do so.
Extra missions like Poison Core, Grinder, Relaxed, etc. count toward this achievement as well as the default Story and Story Challenge.
Look here for examples on how to achieve a gold medal on your story missions