GMCH stands for graphics and memory controller hub. The memory controller and the graphics chip of the 32nm Clarkdale CPUs are not on the CPU die but on a seperate 45nm which is why you see a second temperature.
The GMCH temperature reading may simply be inaccurate. The temperature diode in there is not made to give you a temperature reading but mainly as a way for the chip to know when it gets too hot, i.e. above 100+C.