Obviously, as the description states, you need to get 70 gold medals in (different) Missions and Strangers and Freaks.
However, some people are reporting that it unlocks when they have 69 gold medals, and some are reporting that it unlocks on 71. So if you have not unlocked it by 70 gold medals, just get one more and it should unlock.
The total of 70 Gold Medals can be a combination of both the main storyline missions and Strangers and Freaks missions.
For example: you can have 40 Gold Medals in the main storyline missions and 30 in the Strangers and Freaks missions.
As long as you have a total of 70 Gold Medals, that's what is required.
The objectives DO stack, so you can achieve the gold medal through means of multiple runs (accuracy/headshots one run, speed run next time for time)
However, Rampage missions do NOT stack, which means that you have to get all objectives in a single run in order to achieve gold. - Thanks to @SteInMaN8vSp360 for confirming this.
As for the specific missions and tasks, it is useful to see what the medal requires before starting the mission.
For objectives that require accuracy, headshots and time, aim to first get the accuracy and headshots by only killing enemies with a headshot. Hit these well and you should get good accuracy too.
After this, go for a speed run, skip all cutscenes and just aim on finishing it as fast as possible, forgetting about headshots and accuracy.
For story lines that have multiple choices (the heists), look at which option would give more missions.