It looks like Gauck is a right wing war monger - nothing you would expect from a former priest.
No, you cant break international law, to fight brutal regimes. - the question would show up, who desites if a regime is brutal or not. And usually does a war like intervention creates more harm and problems as sutch a brutal regime. - just have a look at iraq.
And finally would every regime has to fear to get attacked, without any legitimation. This would make the regimes even more brutal, to ensure their surviving. They would use every tool, and different to syria would they use their weapons of massdistruction.