Gold medals are based upon beating a certain score.
The main factors affecting your score are:
1. How fast you complete the mission
2. Your kill/death ratio.
3. Skulls
Difficulty strangely enough does not affect your score, other than making it easier to get a high score on an easier difficulty!
Some skulls will lower your score if they make the game easier, whereas other skulls will give you a bonus to your score if they make the game harder.
Probably the easiest mission to get a gold medal on would be the first mission--just speed through the mission on Normal(more enemies spawned), try to save your units, and you should get it first try.