I think that's
the big advantage of FPGA. If you need acceleration to hit a 10 microsecond latency target, FPGA is what you need. If your latency target is like a millisecond or longer, then GPU can handle a lot more throughput. But GPU can't typically give you a 10-us guarantee.
Okay, bit-banging is another advantage of FPGA that GPU doesn't do as well. There are a few things.