When i try to start BATMAN on the Gateway with this
batmand -g 30mbit/2mbit
the clients will create a gate0 interface. I do not need this gate0
interface. All routers have static public ip-addresses assigned. The
once that i need is, that BATMAN inserts a DEFAULT-ROUTE to the next
neighbour hop thats reach the gateway.
I don't understand why you have the feeling the gate0 interface is a problem.
As Elektra explained choosing a default gateway by "inserting a default route"
is meaningless (in a mesh). If each client should be able to choose its own
gateway there is no way around the tunnel mechanism.
If you dislike the masquerading (which is activated per default) you can learn
here how to disable it. If you really want to have the plain default route
I suggest to use the --policy-routing-script  option to modify the routing
tables on the fly.