There are 116 total missions in GTA V, between Story Missions and Strangers and Freaks. To get a gold medal, you must complete all the mission objectives. You don't need to do all of them in the same run, you can do them one at a time if you prefer, although that would take a while. You need 70 of those.
You keep all your current weapons and skill when you replay a mission, so it would probably be advisable to try to unlock this trophy after you complete the main story, as your arsenal will be way better.
We recommend you do the Stranger & Freaks first, as they are easier than the story missions.