A GFCI CAN be used as a disconnecting means... that is what is supposed to do. This is not up for debate. Whether or not it is acceptable practice is the question.
A little more detail would be helpful, otherwise we'll be answering your question on a lot of assumptions.
There is a circuit, starting with a GFCI at the main building, which then runs to the out building?
Depending on the specific situation and the wiring in the outbuilding (the fact that it is only one circuit), I could see it being passed by an AHJ.
Last edited by NickD; 10/08/13 10:23 PM.