You mean overclock your AMD GPU not Monitor correct?
by saying overclock your Monitor you are referring to increasing/decreasing your Monitor's Resolution in that case the Warranty of your GPU card won't be affected.
But if you meant over or under clocking your GPU Card than yes your AMD Warranty would be voided. But if your GPU card is made by a 3rd party manufacturer you will need to look at their Warranty to see if it will be voided. Most likely it would also be voided under warranty.
Any GPU or CPU settings that makes the GPU/CPU go past or above its safety operating range parameters will void the warranty.
When you overclock your GPU/CPU that is what you are doing.
