After
Nebula is injured during the massacre of the Dorcaernir,
Gamora secretly takes her to Dr Mandibus, a black-market doctor opperating out of
The Barrens, to be healed, hoping she can prevent
Thanos from discovering Nebula's failure, and protect her sister from punishment.
However, Thanos did learn of this, and both Gamora and Nebula were severely punished.