Gradient Seven was a detainment moon that was used by the Galactic Empire during the time of the Galactic Civil War, to house prisoners. In 1 BBY, the Imperial prison barge Purge was sent to transport prisoners to Gradient Seven, although it never arrived there.
Jason Fry, co-author of The Essential Atlas, indicated in June 2015 that Gradient Seven was excluded from The Essential Atlas Online Companion because it was a military code name.