There are 120 possible Gold Medals in total which can be earned: 74 through main missions and 46 through Strangers & Freaks missions. The 74 does include optional family missions for Michael as well as the two alternative finale missions. After you have chosen and played a final mission, you can replay the two other options (from the menu), to get two more gold medals - don't worry, it won't change your save-game in any way.

The best part is that you don't have to get all of the requirements for a Gold Medal in one try. You can replay missions and get all of them step-by-step. (The only exception to this rule are Trevor's rampage side missions - the requirements have to be met in one try, they do not stack.)

Four missions are automatic Gold: the Prologue as well as three optional missions for Michael from his family members, which are triggered through phone calls - don't decline them.

The requirements are hidden before you start the missions the first time. If you want to read about them before doing a mission - possible spoiler alert - you can check them out here: Gold Medal Requirements Guide.
Be aware, that some optional goals like mission time and weapon accuracy only count, if you don't die during the mission - reloading the last checkpoint will invalidate those goals for the current try and you will have to start over.
A pretty common requirement for a Gold Medal is to complete the mission in a certain amount of time (No problem, if you replay missions, as you should know what to do and where to go. If you replay, make sure to skip the cutscenes, as they count towards the time.). Other popular requirements are getting no/few damage, killing a certain amount of enemies, or getting enough headshots in the mission. Several requirements are very mission specific however, and hard to generalize.

Try as many Strangers & Freaks missions again, as they are usually pretty short and have rather easy requirements. Some of the very first story missions are also fairly easy. Another good opportunity is to get Gold Medals in all of the heist-preparation missions. They are mostly short and easy, and some heists have up to five of those. Don't get stuck on the heists and the later missions too long, because they are pretty hard and time-consuming to get gold.