Seems like an interesting issue. I would possibly reset it and see what the readings are. Restart your MacBook Pro and see if that fixed it as well. Mines says GPU Die - Analog with 37 celsius right now. Hope this helps!
If your rMBP is running off the integrated GPU, it will only list the analog reading. You have to trigger the dedicated card (I used gFX Status) in order for the digital readout to show up. Once you switch back to integrated though, the digital reading goes back to --
had that happen once to me i just switched to discrete graphics and it was fine. Otherwise even with integrated graphics the digital temp is displaying. If there is any sensor issue the macs fan will run full blast from the second you hit the power button and your kernel task will be very high.
If you're running off the integrated GPU this should be pretty standard faire.
I've got a 15" early 2011 Macbook Pro and as I'm running on the integrated GPU iStat can't read the dedicated GPU's temperature and is showing it as running at 128C (compared to the CPU at 38C), which is probably the maximum the sensor can measure.