Well I would try to solve why your gpu is crashing under 100% load, keeping the gpu from reaching 100% is a band aid not a fix. First I would try underclocking the gpu, let it go to 100% and see if it still happens. If you have an old or abused gpu it may have degraded to the point where it is no longer stable at stock voltage and clocks.
To answer your question, you could try gimping the cpu, disable cores or underclock it to create a cpu bottleneck, that will limit your gpu load. Or like said above vsync may work if your gpu doesn't hit 100% load when using vsync in whatever game you are playing.