One Day

This is an Achievement. Yes, indeed...you have to play Garry's Mod for 24 hours. This is kind of stupid because a real one day is just 23 hours and 56 minutes and 4 seconds, but the achievement demands that you play 24 hours and not less. Ways to increase your gameplay time is to play Garry's Mod instead of: doing your chores, going out with your girlfriend/boyfriend, and stop using your medication. Though, an easier way is just to leave Garry's Mod on while you do your daily life.