If you are downloading a lot from the Internet to the gaming PC as well as traffic between it and FreeNAS, then you might need two NICs on the gaming PC to take advantage of the dedicated link - assuming its storage system can cope with both gigabyte sources at once.
As someone said, it may be simpler and cheaper to use just one FreeNAS NIC and see if you actually have any practical limitations. Just because you have two NICs on the machine does not mean it is necessary to use both! If your Internet connecting router has a few ethernet ports you don't even have to buy an additional switch, so this is also a potential saving. I doubt you will see the difference for the great majority of the time.
As someone said, it may be simpler and cheaper to use just one FreeNAS NIC and see if you actually have any practical limitations. Just because you have two NICs on the machine does not mean it is necessary to use both! If your Internet connecting router has a few ethernet ports you don't even have to buy an additional switch, so this is also a potential saving. I doubt you will see the difference for the great majority of the time.