yes B is critical because of SS relation to G. and yes, critical activity is determined as you say. and here is also B finsih and Late finish date is same. because program thinks B's duration of 2 days as a fixed unchangeable thing.

but as you can see there is a 1 day gap between B end date and D start date. in other words, lets say B starts, and while performing B, something happens and B slows down and finish by 1 day delay. it will still be ok, no delay to the project will occur, although B is critical. Because B is only critical because of its start date, and NOT its finish date. and where is this information reflected? I assume nowhere?