It could be either or both.
What is the purpose of the fan? To remove heat from the processor (GPU or CPU).
Making the processor work harder (ex. running a game) = more heat = more fan speed to attempt to remove that heat.
How is the overall airflow through the case? What other fans, or blockages, are in there? Are you moving air through the case, as opposed to simply blowing it around?
Ok I'm using the fractal define r3 case. There's 2 other fans. 1 at the back and 1 at the front. I would say the overall airflow is ok because i don't hear an increase in fan noise when doing any other task i.e. watching hd videos.
i'm pretty sure it is the GPU and i think it's being pushed to the limit (no it's not over-clocked) and it's not that great for modern games.