Most network cards will work. For 1g, go for Intel e1000 cards. For 10g, Intel ixgbe or Mellanox ConnextX-3 and up should be fine. For 25g, 40g, 50g, or 100g, go for Mellanox ConnectX-5.
Storage controllers will be a limitation. I would not expect the Protectli Vault's storage controllers to have drivers in GAiA. Check Point's 3100 and 3200 boxes use Atom C2000-series processors with integrated storage controllers. The 3600 and 3800 use Atom C3000-series processors. Something based on either of those lines should work well, and would allow a lot of ECC RAM. The Xeon D-1500 and D-2100 lines use the same storage controllers if you want a little more processor performance.
Be aware a lot of Atom C2000 models are subject to a hardware bug called AVR54 which will eventually render them unbootable. It's fixed in some models of C2000, and not present in any model of C3000 or Xeon. If you can, I'd go for a C3000 or Xeon D-1500. These chips are also good at virtualization (and they support *tons* of RAM) if you later decide you don't need dedicated hardware just for firewalls.