Your GPU will usually always work as hard as it can to produce as many frames per second, regardless of game settings.
Lowering graphic detail in games will probably not affect the GPU's running temperature. But if your GPU is more than capable of producing a smooth image (30-60fps, whatever you think is smooth enough) then you can impose a framerate cap to your game so that the GPU will only work hard enough to produce the set amount of frames per second.
Other methods to reduce your GPU's running temperature:
> Improve air circulation in your GPU's enclosure, like installing more enclosure fans
> Run your computer in an air-conditioned environment
> Modify your graphics card's fan cooler to a better one