D is a native code language that is far superior to C++ and plays well with C (and thus C++). Given that D subsystems can offer C linkage entry points, just as C and C++ can, it would far better to have D rather than C or C++ native code subsystems.
It might be a better language, but we can’t dictate what Gradle users should use. There are no plans to support it at this stage, but no decision not to support it. Language support is driven by demand and contributions.