Are there any significant speed up in time on cracking an RSA key (either brute-force or factoring with general number field sieve) using a GPA or FPGA compared to a CPU? If there is a speed advantage, are there any figures on how much it is?
FPGAs are not like CPUs or GPUs, and cannot be compared like that. Your question lacks sufficient details to provide a meaningful answer. FPGAs come in wildly different sizes and offers paralellism only limited by the logic resources of the FPGA. A giant €100000 FPGA will have way way more logic resources than a €1 FPGA. There are no €100000 GPUs or CPUs.
You must include other metrics to make a meaningful performance comparison. Like investment cost, power usage, implementation effort, and so on.
If you want a hint of high end FPGA capability: Some of largest FPGAs available can do up to 5-10 trillion multiplications per second.