Gantry's PHP compiler doesn't support extends. It's planned to include support for extends in lessphp 1.4, https://github.com/leafo/lessphp/issues/426, though whether Rockettheme will update their implementation at that time is another matter.
You could simply disable Gantry's compiler and use your own, which is my preference;
http://internet-inspired.com/wrote/tutorials/disabling-gantrys-less-compiler/