Half-Life 2's game-play length depends mainly on whether or not you mind watching conversations in-game or running ahead while NPCs are still delivering their lines. It is suggested that you take in the story.
Without paying attention to story, 4-5 hours
Paying attention to story, 5-10 hours depending on the player and their devotion to Achievements/Trophies/Steamworks Medals (ie. see Lambda Locator achievement)
(Times are roughly estimated as I've played through it both quickly and taking my time)
Chat with our AI personalities
Half-Life, Half-Life: Blueshift, Half-life: Opposing Force, Half-life: High Definition pack (might not classify as a game), Half-Life: Source, Half-Life 2, Half-Life 2: Episode 1, and Half-Life 2: Episode 2. Eight games total not including countless mods created by fans that you can download for free on moddb.com and fileplanet.com. Other games by Valve software include Left 4 Dead, Left 4 Dead 2, Portal, Portal 2, Counter Strike, Counter Strike: Source, Team Fortress, Team Fortress 2, and Alien Swarm. Another Half-Life game would be Half-Life: Decay. You might also be able to count Half-Life Deathmatch and Half-Life 2 Deathmatch.
Only for the half life 2 version of garry's mod.
Gordon Freeman
No it is not. It requires one of the games in the half life 2 series such as TF2 portal or half life 2.
By co-op you mean mulitplayer. Yes. Half-Life 2: Deathmatch is only on PC.