I do agree that iStat Menus is one solution, since it can tell me the usage of GPU and memory of Graphics Card.

I am looking into the causes of a high temperature. If iStat Menus tell me that my GPU runs at 200 degrees, I know that my GPU is running at its limit, but I still don't know what's the cause of it: GPU Speed or Memory.

I do think iStat Menus would work. However, I installed iStat Menus on my computer before and the trial period has passed. So it won't work for me now. I'm hoping to get some theoretical answers: e.g. what would happen if the GPU Speed is the bottleneck and what would happen if the memory is the bottleneck.

It seems to me that Intel Power gadget gives you info on Intel-related stuff, like the CPU and Intel HD 4000, but not info on Nvidia GeForce.

smcFanControl gives you the temperature and fan speed, but it still won't let me know what's the cause of this. Is it GPU Speed or Memory Usage.

Yes, I did. It shows that CPU usage is pretty low as always, but it doesn't show any GPU-related info.