Why the G3 battlecruiser was tied to a 16.5-inch gun

Was the G3 battlecruiser intended to use 16.5-inch guns?

Yes. The design was associated with a British 16.5-inch/45-calibre program that followed earlier heavy-gun work.

The intended G3 armament belonged to a sequence of British large-calibre experiments. The 16.5-inch/45 program was distinct from a later 16-inch effort that drew on tests of German guns. It also followed earlier British 13.5-inch/45 and 15-inch/42 development. This sequence shows how post-Jutland gun research fed into the G3 proposals.