Another option I've seen on some of the Hunters was a fake backstay, which was used as the antenna. Since Hunters use a mainsail with a large roach, just have to make sure the sail will clear the fake backstay.
I usually warn people that are thinking about a GAM that there is one issue to consider. The GTO-15 feed line wicks moisture and can corrode out. The antenna then has to be taken down and sent back to the manufacturer, as this part is not replaceable by the user. If you are close to a port where this is no problem, fine. If you are doing extended cruising, something to consider. Other than that, the antenna works great.