"I saw the war of Troy, World War Five and now I'm gonna die in a dungeon!" the Ninth Doctor told Rose and then pulled a face, "In Cardiff."
The Gelth reached into the cell trying to reach him and Rose. The dead Gwyneth stood in the archway. One Gelth came to the opening in the Rift but did not go all the way through. Oswin, who had lost her physical form along with all the other Gelth as a result of the Time War, saw the Doctor and Rose trapped in the dungeon. She saw them grip hands as they prepared to go out fighting.
Oswin thought that her fellow Gelth were incredibly stupid since they were reaching through the bars separating them from the Doctor and Rose with physical bodies. They could get to the Doctor and Rose easily by leaving their hosts. The pair were lucky that Oswin wasn't like the other Gelth or she might be trying to possess one of them right now. Oswin examined Gwyneth's still standing corpse. She had never actually possessed a dead body before but it was the only way she could banish the other Gelth from Earth.
Charles Dickens entered the morgue and used gas to suck the Gelth out of the bodies they were possessing. The Doctor urged Oswin, whom he still thought was Gwyneth, to close the rift. Oswin did not have enough experience at controlling bodies to do what the Doctor asked so he ordered Charles and Rose to flee and prepared to blow up the basement instead with matches that Gwyneth happened to have in her apron. He told Oswin to leave. Oswin let the Doctor feel for Gwyneth's pulse. The Doctor realised Gwyneth was already dead and fled.
As Oswin lit the match she considered going back through the Rift just before the morgue exploded. She could save herself from a fiery death if she was fast enough. Then again if she did, the Gelth she had betrayed would just destroy her anyway. Besides it sucked not having a physical body of her own. For the first and perhaps only time in any of her lives, Oswin had a good idea about how the Great Intelligence felt.
Oswin blew up the basement and allowed her ghostly form to be obliterated in the blast.