GDDR5 is history. Or at least it will be history, as HBM, High Bandwidth Memory, is able to provide at least two times more throughput than its predecessor. GDDR5 is over 7 years old now and has the memory chips attached directly to the video card’s PCB, around the GPU. If you needed more GDDR5 memory, then you would need more such chips inserted in there and more connections to the GPU.
HBM solves this issue by stacking the memory chips on top of the GPU package, the area that holds the GPU, so it doesn’t need anymore extra space than the GPU already needs. It stacks the memory in 3D RAM dies right next to the GPU which makes communication with it much much faster.
With 4 stacks of HBM you can get about 1TB/s transfer rate. Right now only AMD Radeon R9 Fury series has this type of memory, but many other video cards with HBM will be available soon.
Learn more about HBM memory from AMD.