No it's fully used. You can check that in System Properties, Also they are working In Dual channel or Quad Channel mode depending on your MOBO.
Ganged VS Unganged is a pretty long story but the recommended is Unganged. You're fine.
Your system is fine. Ganged vs unganged refers to the method the twin 64-bit memory interfaces work. Unganged has been the default for a while now and it simply means that the system does 2 discrete 64-bit memory operations at once, instead of a single 128-bit memory operation in ganged mode. Ganged tends to perform a bit better in synthetic benchmarks with a single memory heavy thread, but unganged performs better in the real world.