In reply to Jonty Craig:
I guess you are on the 22:10 Ryan Air flight to Stanstead? I would aim to be at Gardermoen by 21:00 at the absolute latest - you need to check in, get through security, walk down to the non-Schengen area at the end of the terminal and go through passport control. Probably no queues at that time of night but you never know.
Otherwise, it seems to be a case of - if I wanted to go there, I wouldn't be starting from err... there!
If you can't get a lift out of Rjukan then it looks like you are tied to the 15:24 bus and about a 100 stops between Rjukan and Oslo S. Then you still have to get to Gardermoen - 20 minutes on the airport express train plus waiting time.
If you can get a lift to Kongsberg you can take the L12 Eidsvoll train (one departure every hour, stops at Gardermoen).
If you can get to Drammen, the airport express train departs every 20 minutes and takes about an hour.