Although certain thermal pastes may be marketed as CPU or GPU pastes exclusively, it’s all actually the same stuff.
The truth is, there’s just no need for the paste to differ in any way. It’s doing the exact same job no matter which bit of hardware you’re applying it to.
It forms a thermally conductive bridge between the baseplate of your CPU and GPU in order to maximize heat transfer between the unit and its cooling system.
So, if you’ve got some CPU thermal paste going spare, and you think your GPU could do with sprucing up in that department, feel free to do so.
As long as you remove the old paste safely, then apply the new layer correctly, it should knock at least a few degrees off your GPU’s running temperature — hurray!
However, it’s important to bear in mind that while no thermal compounds are made for a single bit of hardware, there are different types available.