It's not uncommon to have the gateway outside the subnet, at least when talking about internet; local networks are a lot different.
This kind of configuration is correctly supported on all major OSes. Windows supports it without extra configuration, and Linux also supports it, you just have to tweak the configuration a little bit.
Don't know bot to deal with IP configuration in FreeBSD, but in Linux, you just have to add a route to the gateway before adding the default route.
Looking around, I found this post:
https://forums.freebsd.org/threads/p...etworks.26945/
Maybe it can help you.
The other part, about needing two NICs, that's part of the server configuration. If you needed a server with two NICs (vRack), you should have looked for one which has two NICs.