I found this easiest to do with "Political Realities in Armadillo", the first mission from Marshal Johnson. You can replay this at anytime from the pause menu. Just go to stats, then mission and select it. There are three categories that you must do well in to get a gold. (Time, Accuracy, and Head Shots). These three DO NOT have to be completed on the same run through of a mission so you can replay it several times.
Here's how to accomplish each category
Time: Must be 2:34 or lower.
Skip every cutscene with the "A" button
When the mission starts tap "A" repeatedly to run ahead of the marshal to the saloon.
After skipping the scene, whistle to your horse and get on it as fast as possible and then race to Pleasance House, the marshal will keep up.
Shot everyone as fast as possible (use Casual targeting mode) and I found it fastest to just shoot the main guy in the head instead of the leg.
Accuracy and Headshots: 100% and 5 headshots
These can be done on the same run through. Very simple, just make sure that you use Dead Eye to line up the headshots.
*Edit: Thanks to everyone who confirmed the time.
**Edit: Changed time again (lol) to 2:34 or lower. Thanks again guys for the times and all the positive feedback.