Using with ONE LAN interface GATEWAY

SFOS follows the principle of Zones. So you can have one interface in one zone. 
Do you use VLANs? If not, how did you use it with UTM ? Because this opens a lot of routing questions. 

Essentially you can use it with VLANs easily. 

If you have only one LAN interface without VLANs, you need to get the routing straight, which can be painful. 

Until now no VLANs.

At UTM thats all:

maybe the SFOS needed a LAN and WAN Interface.

Until now no VLANs.

At UTM thats all:

maybe the SFOS needed a LAN and WAN Interface.

And how are you routing "on this firewall"? 
Or whats your outcome here? 
Because yeah, you can do this on SFOS too, and it will work. But how does a client interact with this firewall? Is the default GW for your client the 192.168.0.200? 
Essentially: You can get this working on SFOS too, but not sure if this is really something worth following up. VLANs and a clean cut of interfaces are more applicable. 

You would have to MASQ the traffic on the same subnet and send it weirdly in the network from a to b. 

And you would do this by using a static route. Simply build a static route for 0.0.0.0 to the 192.168.0.25. 
Do not forget the MASQ Rule. 

Thank you - I will try it.

It seams the UTM needs NO gui-configured routing... it routed all to 192.168.0.25 only with the configured GW at NIC level.

yes .200 is the default gw for some clients at home. Until now no vlan at home is needed - maybe the children will do some hacks at layer2-x level ...

Also .200 is an 'exposed host' via german fritzbox; so I can .200 use with ssl vpn from external or do some NAT from external through the fritzbox.

SFOS only allows a standard gateway to be placed on a WAN interface. Doing a WAN interface has other implications like device access acls and certain mechanism are not working. 
Therefore its easy to work with a LAN zone and using 0.0.0.0 route to your GW instead.