In GTAV, each mission has a set of optional objectives to complete to achieve gold on the mission. Are these required to receive 100% game completion? Do they all have to be completed in one run through the mission, or can each objective be completed in separate replays of the mission?
If you login to the rockstar social club and connect your account, you can see the progress you have made toward 100% completion. From what I can tell from the site you need to complete all 69 story missions, 42 hobbies and pastimes, 14 random events, 20 strangers and freaks, and 15 misc missions. Once you are logged in you can see your progress here: http://socialclub.rockstargames.com/games/gtav/career/checklist