A single core can hash (checksum) 5 GB/s using murmurhash. The data you checksum is probably already in L1/L2 cache (write to RAID) or going to be used by userland, and us reading the data will just mean userland process gets its data from cache instead (read from RAID). You can get maybe 2-6 GB/s to GPU. Add the latency (sync, etc.) and GPU time to calculate the hash, you've probably radically slowed down the process. Additionally, assuming DMA transfer, your memory subsystem is more stressed due to both CPU and GPU reading same data.

Oh, and simple xor? Well, assuming data is in L2 already, Intel i7 can xor 10+ GB/s using just single core, between 3 buffers, aka minimum RAID 5. Fastest RAID adapters can achieve only a fraction of that speed.