No it's not. GPU's are a memory constrained ATM, and  using scrypt with the Keccak hashing as used in yacoin only increases the memory used and thus the gap between CPU and GPU shortens but GPU will always be faster. The problems some people is having with GPU implementations is just because of the higher memory usage of yacoin mining, and this usage increases in time.
Keep in mind that when litecoin was released it was a CPU mining coin.