They target cities, but in the entire game are neutral,
Is there a way to go to war?
That is the samnite war scenario, not ager gallicus.
In the samnite scenario, senones can become allied with the samnite by a samnite option and thus entering war (beginning in November -296, costing 50 VP and 10 EP, but cancelled if SPQ controls Faesulae, Arretium, Volaterrae and Volsinii). There is currently no SPQ option that allows to declare war to the senones (I guess for historical reasons ?)
PS: but these regions should not really be Roman objectives if not allied with the samnites - will fix for patch