Kolkokron was an uninhabited planet with little more than rocks and boulders on it. The Fifth Doctor took the Gravis there to separate him from the rest of the Tractators, (TV: Frontios) after a brief unintentional stop on Queth. (PROSE: Life After Queth)
Professor Stream retrieved the Gravis and brought it to 20th century Earth to use for his Quantum Gravity Engine. (AUDIO: The Hollows of Time)