Gabriel and Tanith were two beings indirectly created as the embodiments of the suffering of the universe. They "generated entropy" and caused death merely because they could and sadistically enjoyed themselves.
They created their bodies using pieces from other creatures, until they became beautiful and flawless human beings. They acted like brother and sister, but sometimes kissed and behaved differently. Anything done to them, such as being shot, would let them be "reborn" by the experience; their bodies and clothes repaired themselves and they were essentially immortal.
They were capable of doing anything as they drew their power from the structure of reality itself. This included pulling individuals from alternate timelines and making the Doctor's TARDIS perform calculations for them. Gabriel later tortured Jane Page until she became blind. They also knew of Ace's arrival on Svartos and questioned her as to the nature of time travel, explaining that every time-traveller was responsible for countless genocides: "Every time you blink, you negate all futures in which you didn't."
Eventually, on Cathedral, they abolished/are abolishing/would abolish time and space, leaving Cathedral to be their kingdom from which they would rule, making both everything and nothing mean anything. However, they were turned mortal, and Ace killed Gabriel before chasing down Tanith and killing her as well. (PROSE: Falls the Shadow)
Behind the scenes Edit
Although the cover art depicts Tanith with a short haircut, the novel describes her as possessing shoulder-length hair.