If you want to monitor your nVidia GPU Temperature at all times, this is the extension you are looking for.
It uses nvidia-smi or nvidia-settings as default -if available-, and displays the GPU temperature on the right corner side by side with a nice integrated icon. This extension is available for Gnome Shell 3.6.
It would be nice if you could launch nVidia Settings from there, but you can’t. The extension is made by baco and you can download it from: https://extensions.gnome.org/extension/541/nvidia-gpu-temperature-indicator/