Image: Bend Studio / PlayStationOfficial screenshot from Bend Studio / PlayStation.
Days Done is the Gold trophy for finishing the Days Gone main story. Difficulty does not matter, and free roam opens up afterwards so you can clean up everything else.
Finish the main story on any difficulty. Some regions close for a while during the story, but they all reopen afterwards.
Keep playing after the credits. A few story-linked missions still unlock in free roam.
It unlocks in For An Outlaw Biker, the same mission as I've Been Waiting for This.