Alien-mad "The Truth" is a zany character, to be polite, and here he wants you to "steal whatever they least want you to get". This involves using the newly unlocked jetpack to intercept a train and steal a piece of cargo from it, an object mysteriously dubbed "green goo" hence the mission title. It's simple enough in theory, simply land on the train, kill the goons and steal the goo. The jetpack was a wonderful addition to the GTA series was was put to good use here. Sadly, it hasn't been seen it any GTA game since. Note, if CJ's fat is above 60%, a special cutscene will be triggered in this mission for added fun. The Truth will call him from inside a barrel, mocking him again and telling him to do some exercises. CJ meekly replies that he's "just big boned" while leaving the area.