Garry's Mod: main story about 9.3 hours; 100% completion about 40.5 hours. These are IGDB player-submitted averages from 7 submissions, not a deadline or a prediction of your pace.
Source: IGDB player submissions
Community playtime data is not available for Garry's Mod yet.
Game Tales keeps what happened between you and a game in one Tale you can keep building.