Portal 2 is the sequel to the original which came out in 2007, produced by Valve and was one of the most anticipated sequels from the developer. It's newest addition includes two player online and split screen co-op.
Game speed completion: Dedicated gamers can complete the single player campaign in 4-5 hours with an open guide. Co-op completion can also be done in 4-5 additional hours. Only achievements that could cause issues would be the two online achievements, "Friends List with Benefits" and "Professor Portal" which both require additional friends or boosting accounts in order to fulfill achievement pre-requsites.
GSL Potential: With the written and video walkthroughs available on the internet, point to time ratio would make Portal 2 a benchmark game for any GSL library. The most efficient use for this game would be between two GSL teammates to double up on the co-op point potential. The single player campaign can be completed afterwards on an individual effort. Portal 2 is not a point stackable game, as most achievements involve level progression. With the assistance of guides, not much personal skill is required since most of the difficulty comes from solving in game puzzles.
What the guides don't tell you: Portal 2 is a pretty well documented game, and with the ability to return to specific levels to simplify point cleanup, most players will only have issues with finding a third player who hasn't touched the game.