First, I've not known an international flight to arrive at the B-gates and then have to go back to the G area for clearing customs (so that doesn't make sense to me). Is it possible that you actually do pre-clear customs in Canada?
Clearing customs in G does take some, but the walking time from G to B is fairly easy, no more than 10 min. and moving walkways all the way.
http://mspairport.com/msp/maps_guides/default.aspx