This is either a power supply feed issue, or your GPU is running too hot. It's more probable that your GPU is running too hot, especially if you made some modifications to fan speeds or cooling or any overclocking, but it's possible with older computers that didn't expect to run software in this year.
If you have a laptop or a prebuilt, look up the BIOS stuff for the product you bought and do a "full settings reset" on the BIOS to make sure nothing is being overclocked. Usually this means Hitting one of the "F" keys during bootup and entering BIOS, and hitting a reset all settings button.
If you, or someone else, built said computer, you want to look up your motherboard instead for this instead.
If nothing works out, and it's limited to gmod, try to enter "max_fps 30" in console to limit how much your GPU has to work to render.
PS: Thread Necromancy is my favorite past time.
