Passive network with more than Unity gain
When you speak of "gain", you are usually referring to either voltage or current.  It is not possible for a fully-passive circuit to have an over-unity power gain but it is certainly possible for it to have either voltage gain or current gain.  Just not both at the same time.

Circuit elements that can cause voltage gain:  step-up transformer, parallel-resonant LC tank circuit.

Circuit elements that can cause current gain: step-down transformer, series-resonant LC circuit.