GDDR5 and GDDR5x are both graphics card memory. They are on board the GFX card PCB and do not go into a slot.
A graphics card uses the PCIE slot on the motherboard and the memory type makes no difference.
System memory does go into a slot and DDR4 has a different pin layout to DDR3 and DDR2(older revisions)
Hope this helps
Brit3149