Why would the story need a Tsunade gaiden?
We already know her basic story as a ninja, with her little brother's death and the man she--presumed to--love. And how she's afraid to let anyone die, and I think of blood they said, because she couldn't save them, and that guys blood was all over her hands when she realized he was dead.
We also know the story about the necklace she gave Naruto due to her story, about how only the true Hokage can wear it.
So, not much more to know about her. That was basically what a gaiden is, only it was shown as memories instead.