A crowbar in this situation is a kind of a lever. One end of a lever is used to apply one's force and the other end transmits energy, force and torque to some load, in this case to a door. A necessary part of this construction is some support, or fulcrum, which in the case of a crowbar is located between ends.
I suppose that Gordon Freeman applies force to a crowbar perpendicularly to it. In this case torque is force multiplied by arm length (in more general case it is must also be multiplied by sine of the angle between them). Also this maximizes torque.
Force is given, and an arm length is close to the total length of a crowbar, which is also given. So the answer is `500N * 2m = 1000 N*m.`