The GA-2 Lens Adapter is a vital accessory for the GR IIIx as it allows you to mount the GT-2 Tele Conversion Lens (sold separately).
When connected, the camera is able to automatically detect this second lens and extend the focal length accordingly.
It is vital that when purchasing the intuitive GT-2 Tele Conversion Lens, you also buy this Lens Adapter to ensure full compatibility.