Since garrys mod is a old game and it requires the power from the CPU not the GPU. In a Community named "perpheads" there is a guy who found out that amd cpus run better for garrys mod and the faster your ram works the better fps you will also receive.
sadly i dont have a good setup but because i fiddel around with commands to optimise my gameplay i have around 100 fps CPU: Intel i5 4460 GPU: GTX 750ti