real world IP addresses for firewall, or non-routable IPs ?

[Aloist]

Hi
I have used a cisco router in the past as firewall, with access lists for the filter roules.
The router had a real world IP address for its internal lan interface.
That IP was then configures as gateway IP for other devices in the network.

Now I am about to set up an Opensense firewall.

Should I give the internal interface also a real world IP, so that I can name it gateway for other devices, or should I stick to the non-routable IPs like 192.168.x.x or 10.x.x.x for the opensense device?

I do not want to access the firewall itself from the outside, only from the inside.