The GT-430 is a consumer level card primarily intended for graphics applications like games. It's performance is fairly low. Consider that a Tesla 2050 achieves 340 Gflop/s on a zgemm, compared to the 20 Gflop/s you are reporting. In a quick test, I get 14 Gflop/s with a zgemm on 2 CPU cores (depends on CPU processor). Using the GPU adds additional overhead in copying the matrix back-and-forth to the GPU, so it is not surprising that you see no performance improvement.