This is only a partial truth. The best overall volatility estimator out-of-the-box is the yang-zhang volatility estimator due to its independence of opening gaps, minimum estimation error, and in the presence of numerous opening gaps it degrades nicely to standard volatility estimation. The yang-zhang estimator is an extension of the garman klass estimator, and so using one you get the other. This is invariant of the underlying you are trading. I would be very, very interested in the math you used to arrive at the parent estimator (garman klass) being more effective than it's improvement (yang-zhang) on any specific instrument. Choosing one estimator over the other really depends on the bias of the sector and available data, not a particular investment vehicle.
There are many mathematical considerations to using a volatility estimator of any kind on rolled data. This exercise is left to the reader. There are many, many papers on various estimation correction calculations you can use as rolled data will be biased significantly over a large enough estimation period.