,
A most interesting question, Peter. I could think of it as a variant ii7 (am7), since it fits that place in the rhythm changes so well. But really the Gb kind of disqualifies it as an A-anything. It is more than a passing chord, since it does not go to C#, or contain a C#. It certainly fits in the 'tension/release' model which Mac mentioned, but I dont' think that quite answers the question you asked.
Hmmm....you got me puzzling now...I know it sounds good, but...