As far as I am aware the Pi 3's CPU is currently not overclockable, its maximum clock speed is set at 1.2GHz.
But what about overclocking the GPU? Is it even possible? And would I see performance gains in applications like playing games?
Yes it is possible to overclock the GPU and yes , this leads to a performance improvement.
Look at following resources:
Please don't forget that overclocking is a gamble , your Pi might not be overclockable , crash and corrupt SD cards.
Active cooling is highly advisable while overclocking the Pi3 :