Halo Wars: Definitive Edition

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.

I think the best way to get a gold medal is in Mission 3.
Try to get the 45 kills of hunters, which is a secondary mission of it. Play it normally without skulls and you will get it.
I am posting this because I tried the other solution which does not explain anything specific (besides rushing on the first mission) and the rest feels like generic tips.