When I get some free time I'm going to write a GPU temperature report plugin.
I haven't decided whether it's going to be an actual plugin or something I'm going to integrate into Core Temp (but will come as a separate DLL file).

Here's the preliminary test:

This only works on nVidia GPUs so far. I use nVidia's NVAPI library they provide on their developers' page.
I'll have to dig around and see if ATi offers something similar. As far as I know they don't, so if anybody has any information to share about it, please do.