Motherboard
You'll notice that the underside of a motherboard typically has many exposed pins. A conductive surface would short those pins, potentially damaging the board. A non-conductive surface might work, but it's safer to mount the board properly. Directly resting the board on a surface has the potential for physical damage to any pins and under-board components, not to mention unevenness from any heatsink mounts etc. on the underside.
Motherboards are designed to be mounted with standoffs, which elevate the board above the mounting surface. The screw holes on motherboards are reinforced for this purpose. Use those if possible.
GPU
This tends to have more variation, as some modern GPUs are fully enclosed with a flat back. Those are largely safe to leave resting on a surface. The ones with exposed component leads (looking like metal pins) sticking out should not be in contact with anything conductive, and preferably should not be resting at all.
Note that it may be difficult to extend a PCIe slot away from the motherboard. High-bandwidth connections like PCIe are vulnerable to interference. Ideally, you would just insert the GPU directly on the motherboard slot.
Storage
We'll start with the easy case: SSDs are typically fully enclosed. They can be safely left anywhere, though airflow and a thermally-conductive surface will aid cooling.
HDDs are a bit finicky, because they do tend to have an exposed circuit board. Again, avoid contacting that circuit board with metal, and if it has any protruding leads (pins) try not to rest the drive on those leads (elevate it slightly).
Other considerations
Airflow
Many components are designed to operate with some airflow for cooling. In your typical computer case, this airflow is provided by the CPU, GPU and especially PSU fans, not to mention any additional case fans, and helps cool chips and heatsinks without dedicated fans. If you're mounting them flat on a surface, you'll want to check temperatures and make sure you don't end up with areas of hot air and lacking airflow.
Hey, Thank you for the super in depth answer, and while i would of course not place any component with an exposed board on metal or some other conductive surface, i think the rubber coating im gonna apply shuld be fine, but again your right i shoudl of course use spacers just to be safe, as for airflow, the "case" i designed is open air but im still adding fans (again "just to be safe" lol ). – Bionett – 2018-05-28T06:11:47.283
Won't you be blocking any airflow under those boards with your rubber mats? – Gerard H. Pille – 2018-05-28T06:48:04.887