Campagnolo considered the capacity of the Gran Sport to be 26t, as stamped on the back of the derailleur. I think if you go above 26t, the upper jockey wheel may start to interfere with the large cog, depending on derailleur mounting position, wheel position and chain length. So it may still shift onto the 28, but it might crunch and be somewhat noisy once there. In addition, the chain may be slightly slack because the jockey cage is being rotated further than where it should be to maintain chain tension. Another factor that will help the Gran Sport shift onto a cog bigger than 26 is having the chain as short as possible. This keeps the upper jockey wheel further from the freewheel, but may also mean having to avoid using the larger freewheel cogs when on the big chainring.