Isint GPU memory external RAM? Or is that a whole different ballgame cause of high clock frequency?
GPU have L1/L2 cache hierarchy just like a CPU. SCRYPT only performs well if it is small enough to fit into on-chip memory. If you have to go off-chip, it's going to be a turd. But yes, most graphic cards have seemingly endless amounts of external memory.
The SCRYPT configuration used in Litecoin needs 128KB, which fits easily into onchip cache in a CPU/GPU, and into onchip block memory on many (not all) FPGA.