Will tytannial get 2800 atk if i flip summoned her when someone book of moon her as black garden is in play?

Yes. If Black Garden is in play, then it only effects any monster Normal or Special Summoned, not Flip Summoned. So if Tytannial was already Summoned and had her LP's halved, then was flipped face-down with Book of Moon, you can Flip Summon her and Black Garden will not affect her at all and she will maintain all of her 2800 atk. You can't do this in the same turn it was summoned though. You can Book of Moon Tytannial, but won't be able to flip her until next turn.