Passive filters can provide gain
No, an LC filter just accumulates energy over multiple cycles at its resonant peak, causing the voltage across the filter's components to become higher than the input voltage. The power that can be extracted (continuously) from the filter will always be less than or equal to the power put into the filter.
You most likely confused &quot;voltage gain&quot; (which is what you were thinking of) with &quot;power gain&quot; (which is what the book is talking about). A passive circuit can very easily achieve significant voltage gain (or current gain) but it can never have power gain. A transformer is a very simple example of that. Mismatched transmission lines can also exhibit voltage gain due to reflections.