I'm in no way any authority on bridgestone bikes, but it looks more like one of their commuter bikes CS or road bikes RB than the MB line, guessing from the late 80's to early 90's in any case. I can't tell from the picture, is that a brake booster under the stays in the back?
A good shot of the seatstays might help but I've never heard of any model named GS from them at all.
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